They think that because Japanese has different words for male and female virginity, which tends to get applied to topping and bottoming.
Because people are taught growing up that "men only penetrates" during sex and are virgins if they haven't, anything else doesn't count.
Just like how some real life men believes "if they are giving bj's or handjobs or entering another man's ass, they're not gay".
Or how some X-tians girls thinks taking it up the ass can keep their "virgin cards".
In a sense, everyone is a "virgin" in some way if they haven't experienced something before.
Am I the only one that thinks that if a guy has only had sex with other men as an uke, that he is NO LONGER a virgin? Why do all these male characters think that they are still a virgin just because they haven't been the seme?