Essential Captain America Vol. 1, gives us the answer, at least for Captain America. He hangs out in the Avengers mansion, looking over old photographs of himself from World War II. Then he has some tea, provided by the Avengers' butler, Jarvis.
You'll note that Captain America is hanging out in full costume. Why must he be wearing his super suit when all he's doing is looking through old scrapbooks and drinking tea?
Answer: He is a superhero. He is always "on duty."
Anyway, it's the third panel that gives us our most revealing glimpse into the mind of the super hero. Captain America is so bold, so powerful, so downright goshdarnit honest that he tells Jarvis exactly what are his plans.
I'm going to go masturbate in the shower.
He's a little more subtle than that, but not much. I'll just take a shower-- single-handed. The emphasis on the word "shower" is telling. It's the verbal equivalent of a wink at Jarvis. You know what I mean by shower, Jarvis. And just in case you don't, I'll throw in the totally superfluous line about doing it "single-handed."
I'm not judging Captain America; he's a superhero and he probably has a hard time building romantic relationships. There is a limited pool of super women from which to draw, and if a superhero mates with a "normal" woman, well, as Larry Niven famously postulated in "Man of Steel, Woman of Kleenex," the results could be disastrous for the woman. And of course, we all masturbate in the shower. But do we announce it to the butler before we trudge off wearily to do it?
That's just part of what makes Captain America one of our greatest heroes. He gets bored, he goes to masturbate. And he's secure enough to tell whoever happens to be around.
I'm going to masturbate in the shower.
Excelsior, Captain America!