The critical distinction here is that your sexual orientation does force you to marry someone of a particular sex. It is perfectly possible for a homosexual (or bisexual) individual to marry someone of the opposite sex, and it is perfectly possible for a heterosexual to marry someone of the same sex. Masterpiece Cakeshop and 303 Creative's owners (to reference the highest-profile cases) refused to service weddings because the prospective spouses were of the same sex, not because they were homosexual. (In contrast, if a black person gets married, that will always be a "black person's wedding", so refusing to service it on that basis would be racially discriminatory.)