Trust. What's to say if someone has slept with a load of people that they wont jump ship the next time a decent bit of meat comes along.
Why would trust be an issue if she had never cheated on her partner? You're assuming that just because she's had X number of sexual partners in the past, that she's unable to be monogamous when a relationship becomes exclusive.
If a girl had had sex with a hundred guys before she met me, I don't understand why that's any of my business, assuming she was safe, etc. Because I wasn't involved in her life. Now if she has sex with a hundred guys after we're together, then yeah, I'd have an issue.