Most high-number people rationalize that by claiming that sex is completely contextual. It means one thing in one circumstance and one thing in another circumstance. Therefore, the exact number doesn't matter.

A guy or girl like this will focus on the fact that (s)he has never cheated on someone, or cheated only rarely when much younger, and even though (s)he has had casual sex outside of a committed relationship, it was still meaningful with a person (s)he was very close with. And even though (s)he has had (potentially numerous) one-night-stands, those don't matter so much because they were "just sex", and (s)he never saw those people again.

But today (s)he is with you, and sex means something every single time, because sex can mean different things at different times and places with different people -- so the number doesn't matter! No judging!

Usually, this situation happens in reverse, with the girl being the formerly promiscuous one. Married guys who have very tame, once-a-month and birthdays-only sex with a very sexually reserved and conservative wife, learning that, holy cow, she was a major freak in college. He had to take her out on 24 dates just to get to that first time, but three years before they met, she was screwing strangers in bar bathrooms in positions she won't try with him because she's not like that any more and it would make her feel slutty.

I don't read about the reverse as much, where the guy has the more promiscuous past and the girl's the one freaking out. Women tend to be more likely to freak out if you bought your last girlfriend flowers every week and took her on European vacations, but you took your wife to McDonalds on her last birthday. The sex isn't as big of a deal for them.