I have heard an expanded explanation of the commandment not to commit adultery to include all sexual context outside marriage,and that makes sense to me. But in case you don't buy that, surely you know what St. Paul preaches in the Acts about fornication. St. Paul is very clear that fornication is not for Christians, and all other sins are not committed with the body, but fornication dishonors the temple of the Holy Spirit. In the historic Christian Church, there is Canon Law that forbids marriage between 2 people who've committed carnal acts together before marriage, because we are to marry a spiritual helpmate, and if our intended helpmate can't keep us from that fall, but falls into it as well, that person will not make a good spiritual helpmate. The role of helpmate in spiritual terms is someone who can help us overcome our sinful weakness. If both people fall to the same temptation, they will not be able to help each other through life struggle against each other's individual weakness. In other words, if you really want to marry this person, keep your relationship pure. Even if you leave out the question of a spiritual helpmate able to help you resist temptation, a carnal start on a marriage erodes trust. Marriage is hard enough without the erosion of basic trust.
