One of the best pieces I've ever seen on sexual morality is by J. Budziszewski and can be found here, courtesy of Touchstone mag.
An excerpt:
The problem is that we don’t want to believe that these things are really joined; we don’t want the package deal that they represent. We want to transcend our own nature, like gods. We want to pick and choose among the elements of our sexual design, enjoying just the pieces that we want and not the others. Some people pick and choose one element, others pick and choose another, but they share the illusion that they can pick and choose.
Sometimes such picking and choosing is called “having it all.” Having it all is precisely what it isn’t. A more apt description would be refusing it all, insisting on having only a part, and in the end, not even having that.
It's an argument based purely on philosophy, not theology, and it's a winning argument. Go read it, especially the section on the "Sexual Landscape," from which the above excerpt was taken.
