The "One Drop" System of Sexuality

In "Straight Dude Seeks Same", Jane Ward claims that heterosexuality is defined similar to "Blackness in the nineteenth century," with a "one drop system," in which "any same-sex sexual experience muddies the waters of heterosexuality at best, and marks one as either an open or repressed homosexual, at worst."

We see this attitude applied to MSM (men who have sex with men), bisexual individuals, and in individuals who identify as heterosexual who have sexual relationships with transgender people, among other places.

Here's what I'm wondering. Is it possible for an individual to identify as a sexuality while willingly performing sexual behaviors that seem to contradict their sexual identity? I'm all for people choosing labels they feel define them, so I personally believe that this is the case, but I was wondering what you all thought and why.

For example, you can be a heterosexual man who occasionally enjoys having sex with men. My reasoning is that sexual expression can be done without being sexually attracted towards that person, but preferring sex with men for other reasons such as bonding, sexual release when other options are unavailable, etc.