See, I'm a-okay with the man legs. I suppose some might find it kind of youthful or effeminate to have that exposed skin, and while I don't think there's anything wrong with those qualities, I don't think it's exactly true in this case. Granted, superhero tights highlight muscles just as well, but it shows off his strong muscular legs, and he doesn't look weak or dainty by any stretch.
But maybe that's it, the costume is somehow suggestive of someone who's younger or weaker, so it's seen as somewhat clashing with the person wearing it. (Which IMO makes it all the more fun and interesting, and probably works in Robin's favour sometimes.)