@Jeiben @DeOneChester Thinking back to my own school days (especially around the ages 14-17), not so sure it's just a manga thing. Everyone was calling everyone else a virgin, even when they themselves were. It's just another insult, and one that targets a teenagers' most vulnerable point at that specific age, so being particularly scathing made it more enticing to use.
If there's a manga specific thing to wonder about, it's why nobody ever counters with "aren't you?", because chances are pretty damn high you get a hit. But that would require any sort of smarts or confidence from a manga character, and let's not get too wild here.