be who you are and say what you like
because those who mind, don't matter
and those who matter, don't mind.

1. If you're Percy Weasley, you might say: "We should make love."

Should implies a debt. Or, according to the dictionary, a: "condition, obligation or propriety, probability, or futurity." It's similar to the Latin "debere." In fact, Italians still use this form.Devo andare.
(I should go.)

"I'm ovulating. We should make love."
"We have 5 minutes left to live. We should make love."
"We're drunk and have nothing better to do. We should make love."

2. If you're Ron Weasley, you might bow your head and look at your love interest through shy, curious eyelashes and offer: "Why don't we make love?"

"Why don't we." As in, why not? As in, give me one good reason. As in, "Are you going to sit there and tell me you haven't been thinking about it, too?" As in, "I'm just going to say 'Why don't we?' so I don't look like a fool if you say no." Hey, it has its advantages.

"The kids are asleep. Why don't we make love?"
"You look underdressed. Why don't we make love?"
"We have an hour... or six... to kill. Why don't we make love?"

3. If you're Harry Potter, you might find yourself with your trousers around your knees and your hair wilder than ever and your lips swollen and still not quite get it, in which case, you might stutter: "Do you want to make love?"

Awww. Verray sweet. This implies that, despite the fact that you might feel like you're going to die if you don't do something, the most important thing is how your love interest feels. It's being tuned in to another's desires.

"You seemed to like that. Do you want to make love?"
"You're giving me that look. Do you want to make love?"
"You're driving me crazy. Do you want to make love?"

4. If you're Draco Malfoy, and you know exactly what you want, and you don't really give a fuck about beating around the bush, you might arrogantly toss your head back and proclaim: "Let's make love."

Well now. That doesn't leave much room for argument. Using the 1st person plural command form is... well, it's just that, isn't it? A command.

"You look hot. Let's make love."
"I'm just going to pump up the Marvin Gaye a notch. Let's make love."
"Enough is enough. Let's make love."

5. Of course, if you're Blaise Zabini, and your most striking quality is the fact that you managed to go five books without a gender, and you're not known for your loquacity, then you're probably going to resort to Chuck Norris standards and give your love interest a look that doesn't even need words.

But if there did happen to be words, those words would probably be, simply: "Now."

Four different ways to say something.

"We should."
"Why don't we?"
"Do you want to?"
"Let's."

And one way to just do it.