Courtesy of Comic Book Resources, writer and sex columnist Dan Savage had some interesting things to say about the prevalent lack of sex in superhero movies. Is this accurate? Is it fair? I'm not a fan of generalizations, so let's pick this sucker apart and examine the whys, whats, and ifs, shall we?
First things first: Is it accurate?
Yes, actually. In the X-Men movies, Jean and Scott are married, and share a nice big bed. (Reference Jean's line in X-Men, "I married a good guy.") Meanwhile, Logan has had zero problems attracting ladies into his boudoir, although they do run the risk of nightmare-caused adamantium stabbings. Rogue's powers prevent her from easily romping through Cupid's grove, but she's alone in that category.
Back in the days of X-Men: First Class, characters like Beast, Mystique, and Magneto certainly expressed an interest in getting busy in various combinations, but Maggers ultimately decided Mystique was a little young for him— at the time.
Iron Man's Tony Stark... yeah. We know.
Captain America certainly expressed an interest in Peggy Carter (before and after Natalie Dormer tried to pull him into an embrace), but that whole freezing-to-not-quite-death put a crimp in their dating plans.
Then there's Spider-Man. Tobey Maguire's Peter Parker was getting ready to propose to MJ by the time Spider-Man 3 came around, so... /clears throat/
On the other hand, there's really no sign in The Amazing Spider-Man 1 or 2 that Andrew Garfield's Peter and Emma Stone's Gwen Stacy are engaging in any hanky panky. Of course, he was rather busy fighting the Lizard. Also, the age of our lovebirds is ambiguous and perhaps not quite 18, so make of that what you will.
But this begs the question:
For starters, they may have more important things to do. Saving the city / continent / planet takes precedence over getting a leg over, thank you very much.
Likewise, they may be single, and not be paired off for the majority of the picture. Most superhero movies include the start of a romance, but not usually its consummation.
Also, it may not be appropriate. These flicks are intended for all ages, or at least thirteen and up. We have R-ratings for a reason. Likewise, superheroes are role models, and while there's nothing wrong with casual sex if they're not in a relationship, it doesn't exactly scream 'hero'.
From a meta standpoint, there's money to be made in stretching out that unresolved sexual tension. The longer you wait to see if Parker gets the girl (for instance), the more movie tickets you'll buy at $12 a pop.
Lastly, and I didn't want to bring it up, but if a spandex love scene is executed poorly, it can look fucking ridiculous. I'm giving the stink-eye to you, Watchmen. Just ludicrous.