As a lesbian, I feel like I should support the "any relationship between consenting adults" argument, but I also read "Under the Banner of Heaven" and it really disturbed me. The Mormon Fundamentalist communities in that book didn't just practise plural marriage -- they were also terribly misogynistic. The women had no education, no exposure to the outside world, and no opportuntities besides marriage, and they didn't seem to even have choices about who they married. Even if the women in plural marriages are of the "age of consent", I don't think there can be any true "consent" in those situations, at least, not informed consent.