I said nothing about the importance to the guy, legitimate or not. I only stated what I thought the answer a typical 20's something female would give when asked that kind of question.the
But to address your concern, if it matters to a person, then it matters to a person and that is legitimate enough. Just as some females are concerned about a potential suitor's past - ie: was he a drug addict, did he ever hit his past girlfriends, was he ever a bankrupt, was he ever married, etc.... a person's past is a legitimate concern if you feel it has any effect on you.
If the guy has no tolerance for females who sleep around, even if he is a hypocrite about it, I think it's only wise he ask her if she has and make his decision based on that. If he can't tolerate it, best he knows so he doesn't get himself involved with her. Now of course, there is all this politics about slut shaming and all that, but politics needs to take a side step when it comes to interpersonal relations because the dynamics of interpersonal relations is far different than the world of politics: people's sexual desires, insecurities, and all that don't follow political conventions of any sort.