I'm not bisexual but I did have a very brief sexual relationship with another woman. My youngest sister is, as she puts it, "queer" and what she tells me is that sexual orientation has less to do with who you can enjoy having sex with and more to do with who you can "fall in love" with. Also, there is a difference between finding other women attractive and being sexually attracted to women as opposed to men. Women check out other women for a variety of reasons the most frequent reason being a sort of "sexual status" check, otherwise known as "cattiness". When I was single and in a situation that called for any sort of social-sexual interaction, for instance, Dave's nightclub example, I would on some level keep track of how attractive I was compared to the other women present. If there was another woman present who I felt was more attractive than me, there would be actions I could take in order to feel better about my status. I could "make friends" with the more attractive woman and thus sort of pool our attractiveness or I could "cut her down" in my mind by thinking something like "Yeah, she's beautiful but she's flat-chested and she's probably a b*tch.". Of course, I am far too mature and kind to think in that way anymore. My point is Dave's wife might have french-kissed the other woman at the nightclub simply because she knew that french-kissing another attractive woman made her look more attractive. It might have had more to do with ego than sexual orientation. I know this is true because, as much as I hate to admit it, I know that one of the reasons I had sex with my female friend in college was because I had a crush on her boyfriend.
"Tell me, what is it you plan to do with your one wild and precious life?" - Mary Oliver