Being bisexual is part of who you are. Not everybody is. It stands to reason that if someone isn't bisexual -- which, in this context, presumably also refers to what genitalia they are attracted to -- then they shouldn't be somehow pressured into feeling bad about preferring one or the other.
The best support in this case is to stress the importance of accepting her position and views, and for that matter her sexuality. I don't think it's right to support in any other way; that would be to validate a position that has lost you a friend and will, I fear, lose you many more in the years to come.
And if that doesn't persuade you, how about this: maybe she wants to have and make children of her own with the person she falls in love with. Even with the best will in the world, the biological process rather forces her hand.