It would depend a lot on what stage the relationship is in.
Firstly, if the relationship is still in the "friends" stage, I wouldn't feel threatened at all - she has every to associate with other men, should she choose.
Secondly, even if it was "serious", If I can't trust her I'd rather find out sooner than later.
Mostly, I'd view it as a compliment unless it was on a continuing basis. I have a healthy dose of self-confidence, anyways.
