Another answer is the Theology of the Body. For me that articulated a lot of the things I was able to discern spiritually but not able to explain logically. It is a thing of beauty. But it does not distill down easily into a few sentences. It requires quite a bit of work to understand a very big and very amazing picture of the whole world and where sex fits into it. I have never heard someone explain it successfully in an hour. So when someone asserts in a discussion that they don't see why this is wrong you end up telling them to go to a 6 hour workshop or read a 400 page book. I know how I would react to someone telling me that. I would accuse them of the Phantom Argument Fallacy. That is declaring the existence of a great argument for your position but not actually providing it so it can be scrutinized by your opponent.
So how do we sketch out the Theology of the Body so we can show very quickly that it an interesting bit of moral reasoning and not just a lot of special pleading? Not something that will convince by itself but something that will spark an interest in some people to dig into this further. I can do that with the problems of protestantism. I now find it easy to make a quick comment that shows how this or that depends on a rather questionable authority claim. I wonder if it is possible to do that in discussions about sexual morality.
So I shall make such an attempt. In as few easy steps as possible I shall try and discern what is wrong with gay marriage or contraceptive sex in heterosexual marriage.
- God is love. That means love is at the very center of existence. When we sin against love we sin in the most serious way possible. More than that it means God defines love. We can't just decide what love means. We need to learn it from God and do what He does. We tend to take any relationship we find pleasing and call it love. Mostly we are just using people. God's love was shown on the cross. It is completely unlimited. Completely devoid of self-interest. Completely focused on the fruit of redemption.
- Sex is a call to love. It ties together a strong physical desire, an intense emotional bond that is very painful to break and the possibility of a new eternal person. These 3 things make sex what it is. Removing one distorts sex.
- Because sex is a call to love like God loves any distortion of it is going to distort our ability to love. But love is the most important thing. Love makes communion with God possible. In some ways love is communion with God because God is love.
I can go on and on but the point is not to.