What is the problem with casual consentual sex between two mature adults?
1. Even with contraception, pregnancy can occur. Abortion is a moral quandary. IOW you end up creating new life, and shouldering the responsibilities that come with it. Children deserve to be raised in the best of circumstances -- a committed marriage of a man and woman (so that they get the benefit of both genders if possible). Sure, it can start out that way and go wrong. But you don't selfishly bring children into the world knowing that they won't have this benefit from day one. IOW, sex is not all about YOU and your partner.
2. Sex is incredibly powerful. It bonds hearts together, and has the power to crush and tear people apart. Lives can be unduly harmed by frivolous sex. This is especially true of women, though certainly it can also be true of men as well. It is better to simply acknowledge that sex releases oxytocin, the bonding hormone, and that the more one has sex wit a person, the more bonded one becomes and the better the sex gets. It is far better to PROTECT the heart, to PROTECT sexuality within the commitment of marraige. It's just common sense, and explains why every culture has had the institution of marraige, even after childbearing years have passed.