Okay, so technically this isn't an argument FOR premarital sex, per se, but here are some reasons why I don't see premarital sex as being a big deal in modern Western culture. Here's why:
In the time of the New Testament, and indeed for much of human history, marriages were arranged. There was no dating beforehand, no "getting-to-know-you" phase, no living together, no time to really do anything. Marriage had nothing to do with love and instead was an economic union between the two families. Sure, arranged marriages could conceivably blossom into love, but that wasn't the main point.
However, nowadays, in modern Western culture, this type of marriage is inconceivable. Indeed, I can't imagine it any other way, but this is only because I've grown up in this culture my whole life. Now, marriage is about getting to know someone beforehand. The ultimate end goal is being together with someone who makes you happy emotionally (and physically), not necessarily economically.
So, with this distinction in mind, it's easy to see why premarital sex was a big deal in NT times. It completely shattered the union between the two families, especially if you had sex with the "wrong person". Whereas now, there is no economic union to worry about. Now, everything is based on love (supposedly) so having sex with your girlfriend or boyfriend isn't frowned upon. There's no shame being brought on the family, no risk of doing it with the person you're not supposed to marry, etc. As long as it's based in love, I don't see the problem.
Also, birth control has significantly improved in the past couple centuries. Now, chances of contracting STD's are pretty slim, IF you practice safe sex. I'm not saying it doesn't happen, but your chances are much lower now than they were in NT times. Having sex then was an unsafe practice. Imagine if we still followed the OT law of not sitting on anything a woman touches when she's on her period. That's ridiculous now, because we have tampons, sanitary pads, etc. The risk is virtually zero of contracting anything if the woman practices good sanitation habits. Same with sex now. We have condoms, birth control, etc.
Now, I'm still formulating my opinion on this which is why I'm awaiting your responses. But as far as I'm concerned, it's like masturbation. I actually seek out things that tell me that practicing masturbation or premarital sex is NOT a good idea. But instead, I get the opposite. Masturbation and premarital sex are completely normal practices, as far as studies have shown. There's really no reason to think otherwise other than outdated beliefs. Please don't mistake me here. I'm not one of those incredibly over-sexed people who masturbates 20 times a day and has one-night-stands with all the women in town. Believe me. That is definitely not who I am. But I simply don't understand how the Church can hold to these beliefs when literally every bit of evidence is to the contrary. Can't the Church abandon beliefs in light of new evidence?