"But what happens if your new girlfriend has a much darker and more sinister secret than having slept around a bit?"
And what, exactly, is wrong with "having slept around a bit?"
And what, exactly, is wrong with "having slept around a bit?"
There's nothing morally wrong with sleeping around if that's your deal, doesn't mean I have to want to date people who sleep around.
(I happen not to mind because i) I have a high sex drive, people who've slept around usually do too so we're a good match, and ii) I'm not very jealous, I'd consider an open relationship with the right person. But there are legitimate reasons to worry about it.)
Personally, I use # of sexual partners as a signal for how successful the relationship will be. It's an indicator of how stable the person is in their romantic relationships.
Of course there are counterexamples, i.e. a virgin who ends up a terrible partner and a promiscuous person who ends up being the love of my life.