Doesn’t that just point out that a lot of adults are childish/creepy? I’m not some mega-successful ladies man, but I can’t think bringing a girl back to my place and disrobing to reveal Batman or Superman undies leading to compliments or enthusiasm unless the woman had some unusual fetish.
In fact, what I find most childish about them on the show is Leonard’s embarrassment about them. In one ep Penny says she found a pair of his in the basement dryer (with his initials on them, no less!) and he acts flustered and denies they are his. The mature way to avoid such a situation, adapted by pretty much everyone who’s taken gym since seventh grade, it switching to adult-style underwear. If you’re going to keep wearing them, the mature thing to do is own up to that choice.