We know that within marriage, any sexual activity between a husband and wife necessarily must end properly. They must, “go all the way”.
An unmarried man and woman begin to sexually arouse each other. The natural end of sexual arousal and activity is full intercourse. They are sinning again, or more, or whatever, if they finish in some other way than if they proceed to full intercourse. Therefore, if unmarried people decide to have sexual relations of some sort, they are morally obligated to go all the way, just as they are morally obligated to not use a condom*.*
I hope I am not missing something obvious in my argumentum ad absurdum.