It’s too bad that adult virgins are looked at as being weirdoes.
Well, let's start with your tagline...
I think people do that because they see it as an indirect indictment of their own choices. They went with the herd and popped their cherry because “it was the thing to do”, “it made a man/woman out of me”, etc...and maybe they didn’t really want to at the time, or they weren’t prepared for the emotional aftermath. So they rationalize it by telling themselves they had no choice, that nobody could have done otherwise.
All at once, here in front of ‘em is somebody who had the strength of character to stand apart from the herd. Being confronted with this reality, that they *could* have chosen differently and didn’t have the guts to make a stand when it mattered, is an emotional trigger that they have to beat down and bury at all costs. So, they denigrate anybody who chooses to do other than what they did.