Many partners do not have to be made. Some do not see it as disgusting. Not everyone swallows, anyway. BTW, how is it immoral?
For many, especially religious Christians, Jews and Muslims, premarital sex is considered something that is against God's will. Since you are an atheist, that won't mean much to you. But for those to whom it does mean something, it also makes a great deal of sense. In my life, almost all I know who've engaged in premarital sex have had less fulfilling marriages than those who have. I trust God on this, and it makes sense to me.