How do I want thee? The many faces of insta-lust

Been doing research. You know, the kind that involves reading lots of cool novels. Specifically, I've been looking at insta-lust. The kind of book where the characters are shagging by chapter two, or even chapter one if they're really into it. Why? Because I'm writing a book like that, and I want to see how other authors do it.

That's what authors do, see. We copy. But in the nicest, most imaginative way possible.

So. Insta-lust, it seems to me, comes in several forms, with varying orders of lameness. Because, hey, on the whole, normal people don't walk up to total strangers and start having sex with them. As authors, we have to motivate this strange behaviour. Here, for your edification, are some justifications I found:

1) We're under a spell, goddamn it.
This is a good one when you're doing paranormals. It's magic, folks!

"A curse! Dang it! If I don't shag you right now, my head will explode/a demon will burst from hell and devour me/my dick will shrivel up and fall off!!"

Or alternatively: it's an arousal spell. "Gosh, I'm so damn horny! Oy! You there! Assume the position!"

Lame factor: I'd say 6 out of 10. This can work really well, or it can be totally pathetic.

2) Me caveman. You my destined mate. Ugh.
"Honey, we're fated to be together. Let's have sex, even though we each think the other is a total dweeb... Ooh, actually, this isn't too bad."

Or, alternatively: "You are totally hot. But we cannot be destined mates, because that would be, like, totally uncool. What? You mean it's TWUE? Aargh! My destined mate is rich, immortal and totally gorgeous with a ten-inch {redacted}!!! Oh, noes!!! I WILL not accept my FATE! Bugger off, dickwad!" etc. etc. until they finally give in six iterations later and do the deed, which of course is multi-orgasmic and awesome because, after all, they're destined mates.

Squirm. Some authors make a really good living writing destined mate stories. I can take them or leave them. But on the odd occasion, they can float my boat.

Lame factor: 8 out of 10.

3) Someone else made me horny, but I guess you'll do.
This is when a character is on the rebound from some past lover and just has to get their rocks off with anything that raises their pulse. Then, of course, they fall in lurve and spend the rest of the book trying to convince the other that it meant something.

Alternatively, the revenge shag. "THIS will teach that cheating bastard. Come here, you hot but obviously vapid stud who can't possibly be good for anything but a one-night stand... Oh. OMG. More! Harder!"

I actually quite like the revenge-shag-that-backfires (snigger). I guess I like watching characters squirm.

Lame factor: low. Maybe 4 out of 10.

So what do you think of insta-lust? Yay or nay? What's your favourite/least favourite kind?