Didn't I see this in the official response to criticism of Sins Past: "Well, it's not out of character because it really happens to some people."
"Some people," yes, but that doesn't mean it's in character for these specific people would do it.
The thing whoever writes and defends stories like this doesn't understand is that superheros really ARE better people than the rest of us. (Their girlfriends, too. It makes sense that someone like Peter Parker would be attracted to someone who's a better person than average.) Otherwise, they wouldn't become superheros. If they didn't have superpowers, they'd be the kind of people who spend a few hours each day feeding the homeless. (In these two stories, they don't even have to be that good. Saying that only the best and noblest people don't have stupid one-night stands is setting the bar pretty low.)