This is preaching to the choir, though. Some may be pro-choice, and although there is a difference, it morally amounts to the same thing, which I'll get to in a minute. The people we need to argue with are the ones who don't think religion has any say in what they do privately, or who don't think religion is of value at all. I've noticed that this argument often leads to the question of contraception, and then to the idea that Catholics are afraid of sex or think it's sinful. The same sort of people think this as the ones who think Wiccan worship the devil, Moslems all have four wives, Jewish people killed Jesus, and other assorted tidbits of ignorance.
Catholics think that sex within marriage is a sacrament.
You probably don't understand what this means if you aren't Christian. A sacrament is a visible sign of invisible grace, according to Saint Augustine. Grace is the absolute Love of God, which we don't deserve and can't earn. We receive it free. So sex is a sign of God's unconditional love for us.