You would all probably like to read and listen to Dan Savage's advice column/podcast.
Savage Love - Columns - Savage Love - Dan Savage - The Stranger, Seattle's Only Newspaper
He addressed this issue before. There are lesbians out there who, every once in a while, want to fuck a guy. He's said many times that when he brings it up he always gets angry letters and calls from lesbians who insist that those girls should consider themselves bi, and more from lesbians who thought they were the only one who felt that way. I've known gay guys who have been with more girls than I probably will in my life, and they still consider themselves gay, not bi.
I wouldn't expect it to be anything long term, she probably wants some straight sex with a decent guy and went online instead of putting up with typical guys at a bar. Then again, maybe she's always liked girls but wants to try switching teams to make sure she's not missing out on something that most other people seem to enjoy. I doubt she'd be offended if you brought it up in a tactful way, and that's probably a good idea since one of the most important factors in starting a relationship is making sure you're both going into it with the same expectations.