The Sex and the City actress Cynthia Nixon caused quite a bit of a stir a couple of years ago by saying 'I fully accept being gay is not a choice for some people, but it was a choice for me'. I was not offended because I got it.
A bisexual person, wherever we swing on the Kinsey scale, DO have the choice whether we have a gay relationship or not. I always get annoyed by people saying someone wants or has a 'bisexual relationship'. You either have a hetero or a homo sexual relationship, if you are Poly you can have both types of relationships but you don't have a bisexual relationship, rather that is an orientation.
Yes, it can be seen as a case of taking advantage of heterosexual privilege but there could be many reasons why a person may feel they have to choose and monogamy is of course a big factor in that. Relationships are often a game of chance and numbers wise you are more likely to find yourself in an opposite sex relationship if people go into a relationship with an expectation of monogamy especially, then they have made a choice. Some people come to the realisation later that they can have both types of relationship in their lives by being Poly and open....and then you get crazy threads titled why Poly women are bisexual