As a 50-something lesbian, there is another point of view I would like to offer. When I grew up pretty much all of us were socialized to think that we were straight. This information came from parents and was reinforced in a number of other ways (e.g., TV, magazine ads, school, church, etc.). As such a young person, when I realized that I liked girls, my initial assumption is that I was bisexual. In that time period in history, it was also (in some circles) easier to indicate that you were bi (essentially normal but somewhat kinky—this is in the context of the times and the mind of a confused teen-aged girl).
It takes a good bit of work on one’s own internalized homophobia to admit to yourself that you are gay or lesbian and then to come out to others. For many of my generation, bisexuality was a temporary label along the way to the coming out process. It has taken me a while to work through my own assumptions about people who really are bisexual because of the era in which I came out. I have discussed this with various lesbian friends of my generation who have a shared experience. This is our short-coming or slow awakening to the world of others.
Many are quite flip about remarks that bisexuals have more dating availability. From the experiences I hear from bisexual friends, it is often a greater opportunity to be discriminated against (both gays and straights can be tentative about it at best).
I also know many of my generation and those a bit younger than I am who don’t date women who identify as bisexual. There is a perceived threat to one’s sense of security. This can include the concept that “you may leave me but now the pool of people you may leave me for is even bigger” and the very real concept that “you may get an STD from a guy and that puts my health at risk.”
I am not asserting that there is logic in the belief system, simply trying to share another point of view.