My SO has always had female friends, he's always been a bit of a ladies man and garners female attention (of the friendly sort, usually) quite easily. But those relationships have usually been in group settings or casual keeping in touch on facebook, etc. There are a few exceptions of long time friends that he may meet up with for a drink if they're in town, or stop in to visit without me, but generally he doesn't hang out with women alone. I think because of that, if he were to meet a new female friend and want to hang out alone with her, it'd send up red flags for me and I wouldn't be too comfortable with it. Only because it's not his usual behavior, or they way he generally carries on friendships with women. I know he feels the same about me - I have a few casual male friends but no real close ones that I tend to spend a lot of time with, or ever did - so I know if I suddenly met a man I wanted to hang out with alone, he'd be uncomfortable with that as well.
I suppose though, as long as I got to know her and I was included at times, over time I'd feel more comfortable with the idea, maybe. But if he just came home one night and said "I met suzy Q at the office last week, we're going for drinks on Saturday night just the two of us" I'd have a tough time with it.