Depending on why she says it but yeah I'd be okay with it. I'd expect she's not living the life of a hermit.
Is she saying it to make you jealous? Is she saying it to showboat how popular she thinks she is? Is she running her mouth about these other guys? Or is she saying it matter of factly to state her position? The first three would be strikes against her in my book, but the last one would be cool with me.
I think any guy who'd freak about a new woman stating she dates other men, expecting some gal he barely knows and just started dating to free herself of all other romantic interests and commit herself to just seeing him would be a rather issue riddled guy. I remember one guy on another site saying just that, that he expected a woman to clear the field to date him, to "give it the best chance of working out". This is the "I only date one man/woman at a time" mentality. I see it as controlling. IOW, the person is really saying "I have a fear that I don't have the right stuff to get a woman romantically involved with me, so I feel I need all the help I can get, which means getting rid of the competition and having her agree to be exclusively involved with me from the onset."