It is a choice for bisexual people, though, and it doesn't make their same-sex relationships any less valid. I think the driving force is actual growing gender

It is a choice for bisexual people, though, and it doesn't make their same-sex relationships any less valid. I think the driving force is actual growing gender equality. There is no longer the assumption that "husband" and "wife" are inherently different roles, nor that women are the property of men. When relationships are presumed to be between equals, it no longer matters whether society can easily sort one half of the relationship from the other.