Not sure what the verses above have to do with anything. But:
What's the Biblical basis for forbidding sex outside marriage? I'm not playing the fool here-- I genuinely don't know. I know most churches consider it wrong, and I personally understand why it might not be a good idea, but I can't think of a passage that actually says, "thou shalt not have sex outside the institution of marriage" or anything like that. The only thing that remotely comes to mind are some of Paul's ravings about sex being evil but less so if it's controlled within wedlock.
When you say "what is the biblical basis" I'm not sure whether you are asking where the Bible says that sex outside marriage is sin, or if you are asking whether the Bible says
why it is. That's where I'm unclear. As to where, here are the ones off the top of my head that say that fornication, homosexuality, and/or adultery (which, as far as I can tell, pretty much have it covered when it comes to sex outside marriage) are sin: Matt 15:18-20, Mark 7:21, Rom 1: 21-32, 1 Cor 6:8-10, 2 Cor 12:20-21, Gal 5:18-20, Eph 5:2-6, Col 3:4-6, 1 Tim 1:9-11, Heb 13:3-5, Rev 21:5-8.
You will notice that a lot of those passages are Paul's writings. However, he does not anywhere say that "sex is evil, but it is less evil if in marriage." Nothing of the sort. He (like Jesus) only condemns sex as a sin outside of marriage.