If a person believes in Christianity or Judaism, that person believes it's a sin. "To thine own self be true." The commandments clearly call adultery a sin, and fornication is called a sin throughout the Bible. For thousand of years, since the days of Moses, this has been accepted truth in these two faiths, even though many people committed both acts. However, since the 1960's many of those who claim the name Jew or Christian have premarital sex. The difference is that they don't accept it as sin. Some turn away from church or synagogue because of Biblical law.
Others consider it a sin, but covered by Grace. It's not my business what others believe, but I think people who talk the talk should walk the walk. Yet, many who have left the church to do their own thing call those who attend hypocrites. It's not for us to judge. As I stated earlier, "To thine own self be true." (That's Shakespeare, not the Bible.) If someone believes it's a sin and accepts the Bible as truth, it must be a very difficult way of life. If not, it doesn't matter what God's law is; they have another faith or they are a law unto themselves. I believe the Hedonists believe, "If you like something, and it makes you feel good it's fine."
I don't intend to argue the point, but I honestly wonder if many Jewish and Christian parents are teaching their children what God's law is. I have heard (really!) girls say, "I'll sleep with you Saturday night if you go to church with me on Sunday." HUH ???? I'm glad they want to go to church, but were they reading a book or writing a shopping list during the sermon? That is the morality portrayed on television, not in a synagogue or Christian church- not a judgment, but an observation.
Religion is too serious and personal to be argued in this type of forum, so I'm done. (It's about time!!) Perhaps someone else will discuss the practical aspects-like single mothers who have to go to on welfare, at least temporarily, because their lovers were interested in them only for sex, or what happens if their lover dies without a will. That might be interesting. As jcat says, the laws differ from country to country, and, I assume, state to state.