Hmmm. Well, that's a new one to me. My gut response is if you know she would never leave him for you, then you would be better off in the long run with moving along and finding someone you won't have to share with somebody else.

Have you seriously talked to her about it, though? She might not love him as much as you think. Maybe she's with him mainly so it looks like she's hetero. At least you would know more where you stand if you bring the topic up to her when he's not around.....