There are interesting ways to find out that two of your friends are now a couple.
There are amusing ways.
There are simple or intrusive ways.
However, the way Naruto found out could be classed somewhere among all three.
He had come to Kakashi's apartment for a very innocent, traditional reason: To inquire if they were still going out for drinks that night. He had proposed the idea two days previous and had received only a vague, indefinite answer from the man. As he had nothing else to occupy his time that particular night, Naruto arrived seeking confirmation of their plans.
Upon arrival, he discovered that, to his surprise, Kakashi did not answer when he knocked on the door. At this, he produced the spare key he had been given after much persuasion. It opened easily enough, allowing him to enter the chiefly illuminated domicile.
"Kakashi?" he called, shutting the door behind him as he ventured inside.
The answer came in the form of a few indistinct, muffled sounds, drifting from the closed bedroom door across the room.
Not utterly satisfied that the man was indeed home, Naruto strode across the room and pressed his ear against the door. For this, he was rewarded with several strange and still quite muffled noises. He almost thought he heard a woman's voice, but that was of course preposterous.
Pushing open the door, he uttered cheerfully,
"Hey Kakashi, are we still on fo-?"
The words died on his lips at once when what was going on inside the room fully came to his attention.
It would have been less shocking if they had least been clothed, but, alas, neither of the pair were. Moreover, the two were not simply lying in bed either, something else that would have produced a more serene reaction. Per contra, they were actually engaged in the act when Naruto burst into the room.
Or, if one wishes to put it vividly, 35-year-old Hatake Kakashi and 20-year-old Haruno Sakura were in the action of having sex when their friend barged in.
As you might expect, his reaction was something like this:
"WHA-WHAT THE...?!"
followed hastily by a string of unrelated, confused verbs and nouns.
Approximately twenty minutes later, Naruto had calmed down significantly.
This was helped by the fact that Kakashi and Sakura, after they got over their own initial astonishment and mortification (and fury, on Sakura's part, expressed by the ring of purple that now encircled Naruto's right eye), had hastened to dress themselves.
Now, all clothed and settled down, two of the three were sitting on the couch while the third paced before them.
After a moment of silence in which Naruto gathered his stricken thoughts, he queried in a tone of forced serenity,
"What exactly were you two doing?"
Kakashi arched his eyebrow at this question, rather puzzled by it.
"Erm, Naruto, I can understand if you missed Iruka's sex education lecture, but you should at least know..."
Sakura dived into the very middle of Kakashi's utterance, cutting him off. She had been waiting for the perfect chance to beat down Naruto's inevitable complaints and he had handed it to her.
"Naruto, we're both adults. It's none of your business if me and Kakashi want to have sex!"
In response to this, the other paused in his pacing for a moment to gaze directly at the female.
"But why would you want to have sex? Are you two...in a relationship?"
The two glanced at each other, neither especially willing to answer that specific query. They would have happily explained anything else under the sun. In their moment of hesitation, the pair were silently debating which one of them would drop the bombshell. In the end, it was Kakashi's reluctant duty.
"Yes, we are, now that you mention it. Is that a problem?"
Looking immensely stern and parental all of the sudden, Naruto uttered in a severe tone,
"Now that you mention it, it kind of is."
He thrust a finger in Kakashi's direction, a very blunt utterance emerging from his lips.
"I really don't think you'd make a good boyfriend for Sakura. You're late most of the time, you read dirty books, you're much older than her, and you USED to be her sensei."
"So what? If I'm alright with those things, isn't that the only thing that matters?"
It was Sakura speaking this time, and in a very aggressive tone. She was gazing at Naruto with a rebellious challenge all over her features. She so resembled a teenager at that moment that one could almost forget she was twenty years of age.
Naruto gave her such a distinctly sympathetic look that she greatly desired to hit him over the head with something large.
"I'm only thinking of your wellbeing, Sakura. I know you probably don't understand at your age, but choices like this could effect you for the rest of your life."
"My age? YOU'RE THE SAME AGE AS ME, YOU IDIOT!"
Passing over this piece of information, the other man journeyed ahead.
"Impulsive decisions like this are the building blocks for failure..."
"Impulsive?! I'll have you know that me and Kakashi talked about it for WEEKS before we went on our first date..."
"...and having sex so early is never a good sign. How do you know you'll stick with each other if there isn't sex involved?"
"We've been dating for over half a year and Kakashi hasn't even seen me naked before tonight!"
"Well, those are insignificant points..."
It could have easily been debated that this was NOT the case. However, Naruto did not pause long enough for any lengthy objections. He plowed ahead, his expression shifting from disapproval to intense thought.
"On the other hand, if you two are willing to give this a try and feel that it's worth it, I might be able to get used to it..."
"Oh really?" Kakashi replied, looking rather irritated. "I had no idea I had to get the approval of someone younger than me before I was allowed to date a woman I love."
He adjoined, his tone shifting to affection,
"I love Sakura more than I even thought possible. I'm not sure if she feels the exact same way about me, but she at least loves me enough to want to be with me in a serious relationship. Isn't that enough?"
"It's definitely touching. But aren't you using the word "love" loosely? I believe in true love, of course, but how do you know that you're in love?"
Sakura looked for a moment as if she was going to demonstrate how much she loved Kakashi by punching Naruto in the jaw for suggesting otherwise, but her lover soothed her temper by lightly touching her knee. This was a handy little trick he had picked up over the course of a few months. A trick that only he was able to do, as it wouldn't product pleasant results for anyone else. It was he who replied to this query, with a serene look that didn't quite match his thoughts. He was truly regretting giving Naruto that spare key.
"I still don't see why you're overreacting."
"Excuse me, but if you went through an unlocked door and shortly thereafter found your former sensei and a girl who was once madly in love with you having hot, steamy sex..."
"Excuse me, but who was in love with who?" Sakura snapped threateningly.
"More insignificant little details... Anyway, are you telling me you wouldn't be at least kind of surprised?"
"Alright, it's understandable," Kakashi reluctantly allowed. "But I still don't understand how it's any of your business."
Passing over this once again, Naruto continued forcefully,
"What exactly are your intentions? Are you going to get married? Have children? Grow old together? Just what are you planning to do together?"
"There are several things we could be doing right now...if you didn't have such bad timing."
"What if Sakura gets pregnant? Are you ready to be a father, Kakashi?"
"Of course he is!" Sakura leapt into the silence before Kakashi could speak. "Right, Kakashi?"
Kakashi twitched, albeit very slightly. Perhaps his body was remembering what had happened the last time he answered that question wrong. Needless to say, despite his strict policy of rarely crossing his beloved, he ended up with an impressive bruise. That was one of the few occasions his method for calming her down didn't work. He hadn't been paying attention when he replied to her idle query.
"Ye-Yes, Sakura."
"Marriage...have you even thought of that?" Naruto continued insistently.
"Several times," Kakashi responded crisply. "Just yesterday we had a long discussion about our future together."
"That doesn't mean anything. The two of you could be setting yourself up for a false start, you know."
A heavy sigh drifted from between Kakashi's lips.
"To sum it all up, me and Sakura love each other, you're unwilling to accept it, and we're both very sorry you had to find out like this. Can you please go home now?"
Naruto tossed the proposal around his mind for a moment, clearly not entirely satisfied with it. A part of him desired to remain, but he had exhausted his most efficient arguments. Left over were only idiotic, childish pleas.
"Fine," he uttered at last. "You win, Kakashi. But if you break her heart, I'll kill you. Remember that."
With this grim threat hanging in the air, he chose to depart.
Left to their own devices, a slightly giggling Sakura enticed Kakashi back into the bedroom.
"Do you think he'll ever accept us being together?" the female inquired in a somewhat annoyed tone.
"First, he'll be in denial. Then, after we've married and had a child, he'll finally have to accept that we're in love and there's nothing he can do about it."
"Seems like a long wait for something that should happen naturally."
Kakashi planted a tender kiss on her cheek and uttered quietly,
"Trust me, some things are worth the wait..."