As a male lurker of this sub (and thanks for all the info ladies), pls. allow me to throw in my 2 cents to provide a diff perspective and possible explanation. It's not the number so much as it is the male ego. EVERY guy wants to be the "best" the woman he's with has ever been with. So, if a woman has been with 10 guys, he's got a 10% chance, if she's been with a 100 he's only got a 1% chance. Honestly, that's most of it. Thinking back to when I was in college, I had a friend whose number was probably 25 or 30 but all one night stands. So even though his number was higher than mine, since I had a gf, I had sex regularly and in a month or two had more sex than he had in his entire life, not even counting gf's in HS and others in college. I was a serial monogamist! :-) By the same token, some women that have had majority one night stands, even if their number is 40 or 50, a guy does have a good chance to be one of the best if they have sex multiple times (takes time to learn what each other likes).
BUT guys don't look at it that way. It's more the % and their illogical rationale of their chance to be the "best." Egos, both male and female, can be a real touchy thing sometimes...unfortunately.