I found this a bit of a tough call. I voted for "heterosexual privilege" since it's being in a heterosexual relationship that allows the woman to marry the partner of her choice while ignoring that same-sex partnerships don't have that right. But on the other hand, for a self-identified bisexual (as I presume from your wording that she is) to "forget" that same-sex couples can't marry is a worse offense, in my opinion, than for a heterosexual to do the same thing. So it seems like it needs a separate term, "bisexual privilege," indicating the ability/choice to have legal recognition and social approval of your partnership choice while still claiming a queer identity. Obviously not all bisexuals exercise this privilege, but it's available to bisexual people as it isn't to lesbians and gay men.