Personally, I believe sex should only happen after marriage. There's less chance that the other will run off after doing it, and there's also would be a lot less teen parents and stuff.
And yes, I know that this argument can be taken down by "CONDOMS, CONDOMS EVERYWHERE." But, quite frankly, condoms are great if you don't want kids or to protect yourself from diseases, but there's so many people out there who immediately associate condoms as "problem free sex forever!" and then they go have sex with a bunch of drunken girls and eventually they get too drunk themselves and forget to put it or it breaks or it was expired or whatever, and then they get a kid, and then they say "it's not mine because if I say that, I can continue having sex with a bunch of people while this poor girl raises her kid on her own because fuck responsibilities" and it all ends up on the Maury show and those douchebags become the people I despise above all.
That, and because, personally, I'd prefer dating and marrying a cute virgin rather than some girl who will compare my sexuality with the other guys she had sex with.