"Premarital sex is like spoilers!"
Which it totally is, because it's like being told the intimate details of the movie, which some people want to know beforehand because they're worried it will suck or they're just too excited to wait, and which other people can't stand to know because they want to experience it all in the context of the actual story.
And because I can't leave a good metaphor alone, I extend it: if premarital sex is like spoilers, then making out/cuddling/kissing is like watching the teaser/trailer/preview. Multiple times. Which we all do when we're really looking forward to the movie--often even if we don't want spoilers. :)