Not because I don't believe them; or because you don't.
But because we've been told for some time neither of these are acceptable grounds for sexual morality--and we can do without them.
That's the reason why we're supposed to accept contraception, in vitro fertilization, and now, equivalency between homosexual and heterosexual activity.
So tell me: what are the criteria for sexual morality, if not, (a) Natural Law and (b) the Bible?
Make your case. Tell me why incest or polygamy are wrong? Or, if you think they aren't wrong, then tell me what the criteria for right and wrong, in matters of sex, are?