If I don't know what my actions will produce, then I cannot make a sensible decision about whether they are moral or not. Ultimately, I think our understanding of neurobiology and evolutionary biology and psychology will reduce our understanding of morality to some well-defined biological constructs.
Iron age scriptures might argue that homosexuality is "wrong", but scientific discoveries about the frequency of homosexual behaviour in a variety of species tell us that it is completely natural in a rather fixed fraction of populations and that it has no apparent negative evolutionary impacts. This surely tells us that it is biologically based, not harmful and not innately "wrong".
The fact that infidelity, for example, is a fact of biology must, for any thinking person, modify any "absolute" condemnation of it.If the first two quotes don't seem problematic to you, the third ought to make clear what's going wrong here. Baggini battles gamely, but I'm not sure he really gets through to his opponent.