I just wanted to get some basic groundwork out of the way in dealing with discussions about sex from a Catholic perspective. I'll do this in several parts, since there is actually a lot of ground work.
The virtuous life is best because it will lead to the greatest joy. This is the foundation of the moral thought of St. Thomas Aquinas, great doctor of the Church, and indeed, of our entire Faith. We do not teach these sexual morals because the Church told us so; the Church told us so because she has discovered they, and they alone, will lead us to greatest joy.
Objection: I don't feel happy. I can hear you say that now, as you look longingly at your spouse, wanting to enjoy marital love but finding yourself thwarted by whatever is in your way. Yes, but happiness is not the same as joy. Joy is the deep abiding satisfaction of doing what is best for your beloved. If by happiness you mean satisfying your every whim, then, yes Catholicism will never help you achieve that. Which brings me to another point.
Getting what you want is not always good for you. To be Catholic is to recognize that sometimes you will want things that will not be in your best interest; a second helping of cake, perhaps, or another drink. Maybe even a person or to watch a certain TV show, to sleep in when we ought to get up and work. There are many things that we may desire, but would hurt us if we had them. This is the opposite of what the world teaches, but it is nonetheless empirically true.
These are the basic understandings you must have in order to engage in a Catholic conversation about right and wrong in any arena, but especially in the arena of sexuality. The modern world teaches us that to not satisfy our sexual desires will lead to mental illness, extreme physical discomfort, and a host of other maladies. Thankfully, none of this is true. People of all faiths have lived by these principles for thousands of years and produced great works of art, literature, philosophy, theology, and science. You can be healthy and not have sex - I promise.