Perhaps it's because people realized that putting off mutual sexual gratification until marriage is just a ploy to get people into marriages with potentially horrible sexual partners and to guilt them with the sin of divorce?
LOL, :yes:
Marriage as a custom was created so to either A) unite powerful families, or B) Essentially a way to "Sell" ones daughter for a few cows, while also ensuring that said daughter would be taken care of financially speaking. "Love" being a part of marriage is actually a very recent social institution, generally speaking the 20th century. There was no such thing as loving the one you married. You actually married the person that could take care of you, and you had an affair with the one you loved as in the case of Lancelot and Queen Guinevere in the King Arthur traditions.
The no sex until marriage thing was probably a result of fears of all the bad things that could happen having sex back then (unwanted pregnancies, STD's, etc etc.) which we don't necessarily have to worry about if you're half way intelligent these days.
So I say if you find a person that you go on a date with attractive, then go right ahead on and get yours hunny. However, if I guy refuses to continue to date you because you don't have sex with him then he probably isn't the type your after anyways. Of course, there is a certain timeframe for this, at least for us "non-abstainers". I give girls I really like a month, sometimes even two, but for everybody else, they only get 3 dates lol.