If you haven't already spoken to her, I'd advise that you do so soon. It does sound like it could be a really awkward conversation - it's hard to have the "I don't see you that way" talk with anyone, but it's weird when you like the person, and just don't see them in a sexual way.

I guess I'd start off by explaining that you wanted to talk about the kiss because you've been feeling awkward / uneasy / whatever about it since it happened. Explain how much you value her friendship, and you don't want one stoned kiss to drive a wedge between you, just because you can't see yourself feeling that way towards another woman. I'd take care to stress how much you like having her as a friend, and that you can't help the fact that you're straight!

I've been in a similar kind of situation, although the girl wasn't my best friend. I had to have that kinda talk with her, but it was even more awkward because she knows I do swing that way. It all turned out fine in the end though, and we're still friends, so I reckon your mate should be fine with it, once you've explained your feelings.

Good luck!