It was after Sherlock had saved Irene's life. No, not Karachi. It was a different situation this time.

They had caught the next plane and flew back to London. Sherlock had originally planned to take her into hiding somewhere in America, but Irene insisted that she would feel safest in London. The entire ride, both were silent, not really knowing what to say. When they made it to her old flat, Sherlock intended to leave her there. He stole a glance at her with which he managed to finally make a deduction from. She was vulnerable, heartbroken, tired, and maybe slightly horny. He couldn't help but let his pulse quicken at the last one, but he made himself ignore it. He put out his hand.
"Well, then, nice seeing you, Ms. Adler."
"You too, Mr. Holmes. Thank you."
He nodded and started to walk away. But he couldn't help but think about his last deduction. She was a sex worker and could get her sexual pleasure at any time and she had to become aroused now? He turned around. She was still there. She smirked. "I took your pulse," she mouthed.
She looked so beautiful in the moonlight. He ran to her and couldn't have helped it, but suddenly, they were kissing. She looked up at him.
"I noticed you were feeling quite aroused," he said matter-of-factly, trying to explain himself.
"It doesn't take a man with massive deductive skills to notice that," Irene smiled as she ran her hand along his arm.
Sherlock didn't notice he was feeling quite aroused too until she took his hand and started leading him up the stairs.
"Let's have dinner,"she cooed as she walked.
He nodded eagerly.
They entered the flat and Sherlock closed the door behind him.
She lead him up the stairs and into the bedroom.
He wanted her and she wanted him.

He climbed on top of her and tugged at her clothing.

They had so many layers of clothing on that by the time they got it all off, both were extremely frustrated and there was no time for slow, simple starters. Before much could happen, though, Sherlock rolled off and sighed.
"Oh, Mr. Holmes," Irene groaned, " What are you doing?"
"It's...I'm not sure if...it's just..it's my first time." Sherlock stuttered.
"You think I don't know that, Mr. Holmes," Irene replied.
"I mean, I still want to. I just...don't want to disappoint you."
"I'm sure whatever you do will be to my liking. If I don't get what I want, I can always obtain it from you," she said with a smile.
"That's the thing, "Sherlock continued, "You're experienced. I just-"
"It'll be okay. The question is, do you want to? If you don't, I won't push." Irene ensured.
"Of course I do!" Sherlock exclaimed.
"Then, we will. But you can make the decisions. Would you like that?" She said, stroking his stomach, wanting to get the mood back.
Sherlock nodded. He felt as she stopped stroking his stomach and lay back down on the bed.
He lay back on top of her. She nodded. He pressed his body against her and kissed her again.
She felt him enter her, hard, fast, and sloppy. But she still loved it. Their rhythm was slow and as she felt herself tighten around him, they came together. Irene didn't make any noise louder than a gasp but Sherlock yelled a little louder than he should of.

He rolled off her again.
Neither said anything.
After a few minutes, they started up again. He knew men could only have one orgasm but he wanted to give her all he had. He licked his fingers and slipped his forefinger into her warm, wet sex. She moaned and caused him to smile. He pumped his finger, again, in a sloppy matter. Irene couldn't take it anymore and guided his second finger into her, too. With this one, she helped him do it properly without the sloppiness. She came again. They went at it all night and both came multiple times.
In the happiness, the two didn't expect this night to lead to a whole new adventure.