Just as there are gay people who don't enjoy being penetrated, there are plenty of straight women who enjoy anal sex. Sexual acts don't define the sexual orientation. Being gay is about more than the sexual act. When straight people focus on the 'act,' it's usually part of a pattern of denigration toward the gay person. It's especially bad when someone secretly harbors a sexual response and their internalized homophobia causes a violent lashing out.
Some people think it's a choice, others think it's genetics.
I believe people are born gay. You just don't 'choose' your sexual orientation. As far as anal sex goes, that's just part of the sexual relations.