Here is Jennifer Roback Morse on "Love and... Marriage and the meaning of sex":
We can construct, deconstruct and reconstruct our sexuality any way we want: it is our privilege as thinking creatures. However, human sexuality has a specific nature, regardless of what we believe or say about it. We are more likely to be satisfied with the outcome, if we work with our biology rather than against it. We will be happier if we face reality on its own terms.
The law of marriage is not the only social structure that creates the context for socially acceptable sexual behavior. But the law does play a key part. This is why it is utterly reasonable for the law of marriage to take into account the natural purposes of human sexuality. And it is utterly unreasonable for the law to treat all sexual unions as though they were equivalent.