I'll just give the Hindu mystical POV, so it's also in this discussion, not for argument or anything more than information. Most people wouldn't know this, and even more wouldn't believe it. The mystical reason for not having sex is to not get hooked to someone that you may not want to be hooked to in the long run. If premarital sex happens for two people that end up getting married then the mystical consequences wouldn't be much at all. Remaining with one person strengthens the bond of marriage.
We all have subtle nerve currents within the astral body called nadis. They extend out past the physical body a few feet, and can be seen by mystics in auras. A the end of these nadis are hooks, and they work in sexuality much the same way velcro works physically. When two people connect via the intimacy of intercourse, these nadis get attached. The more intercourse, the stronger the bond. So couples who have been married a long time are really 'hooked' ... so much that they can sense each other moods and more, if they have any mystical or intuitive feelings at all. So the object of not sleeping around is to avoid the subtle effects coming from other people who you don't really want coming at you. All this makes divorce difficult, for example, and there are subtle effects on very promiscuous people.
It takes a fairly lenghty time for the 'glue' to wear off, usually considered 24 years.
In the Hindu marriage ceremony, astral beings (devas) are ceremoniously assigned to be helpers for the couple throughout their lives, just as the two extended families are helpers throughout their lives. That's also why a lot of consideration is put into a marriage on all sides.
So those of us with deeper view more mystical views would take this into consideration as well in deciding whether or not to have premarital sex, and that's definitely why adultery is really frowned upon. It weakens the marriage, the institution, the family, and ultimately the way of life.