Hmm. Okay, my two bits...
Let me point out a problem with your question. A problem of perspective.
No, not a moral one. Just an interesting viewpoint one.
You seem to be assuming that a) Sex Is Bad and b) Marriage Makes it Good.
This is actually incorrect.
In the Bible record and in the world around us, we see: Animals have sex. Humans have sex. Plants don't, but they also procreate.
So basically, sex and procreation are natural, part of creation.
Sex, in short, is good.
Now, you may be asking,
what about sex for pleasure? I'll get to that.
Anyhow, I'd like to point out that Sex is not bad. God invented it himself. He also told humanity to be fruitful and multiply. For the last, oh, at least 6 thousand years if not more depending on your views on Universal Synthesis (a term that, yes, I did just make up now, meaning the way that the universe came to pass,) before the invention of in vitro fertilization, human beings and the rest of animal/sentient life have had to use sex to be fruitful or multiply... something that if God had a real problem with, he would have mitigated.
Okay, so in general, I think it's entirely appropriate to say that the concept of sex is good. But, what about sex for pleasure?
Basically, Sex is good for more than just making babies. The Bible in fact
Commands married couples to have sex.
Really.
No, seriously really.
Okay, fine, I'll unpack it. The Apostle Paul commands that married couples not abstain from sex unless for brief fasts -- and then only for purposes of prayer. Otherwise, he says, you might be tempted to do bad things with your sex life.
In short: Sex good. No sex... could be problematic, so avoid it.
No, I'm not going to quote the verse. Trust me, it's in there. I gave you a hint, it's in the writings of Paul.
Go find it yourself. Thank you.
Alright... but... the Bible says a lot of bad stuff about sex, right?
Wrong!
The Bible says nothing bad about sex. It says quite a lot about
promiscuity -- that is, doing bad stuff with your sex life.
See, I said sex was good. When I said "sex," I meant specifically sex the way God invented it: Intimate, Close, IN MARRIAGE, and Pleasurable.
Sex outside of marriage, Sex against consent, Sex with Animals, Sex with your own gender... these are all things that are not "sex" the way God invented it.
Sex is good. Promiscuity is bad.
But never forget, Sex is good. Not dirty, sinful, or evil... the idea that Sex itself is a sin is a holdover from the Gnostic heresies, which also teach that God is evil and hates Jesus and the rest of us... please, let's not go there
It's a rant for another time