On the morality of sex

Yes but sexuality is more difficult to controls. Today, it's sexuality that defines if moral is acceptable or not.

No, it isn't clear, but I might be stupid...
Where do the moral problems arise from?

I think it's because sex for the vast majority of people and contrary to what many may claim is accompanied by very strong emotions that prompt people to act in a certain way.

It can be manipulative on one hand and emotionally painful if unsuccessful. Because of this, people want to regulate it some how.

Turns out modern youfs under 23ish are having the least sex of the past few generations- this despite society becoming even more overtly sexually explicit and the concept of detached sex being prolific.

People try to act tough but sex = love, which is basically cocaine so people are afraid of it.

Because emotions
Can't disobey our instinctual programming

Because sex is tied to emotions for evolutionary reasons. Love for your mate to make you keep taking care of her and ensure she'll successfully bear your offspring. Jealousy to make sure they're your offspring and no one else's. Envy to make sure you actually try to get a mate.

Combine this with the fact that if you remove all social constraint, you end up with a situation where the majority of females will flock to the top 10% or so of most desirable males, and you have a recipe for enormous chaos and violence and thus a need to codify sexuality.

That makes sense (though the less young people having sex part can be associated with females flocking to the top males already being widespread, but still).

So basically, sex = emotions and that's what its special standpoint, traditions aside, boils down to and always will?
And in that regard, just to clarify, sex and emotions being bound to one another is not an absolute for all humanity, but still holds true for nearly all of it and that's where its morality arises from?

By now I'm somewhere quieter and could word the OP better, but it's pretty much already been replied.

Thank you to all who replied.

Free love

There are many reasons:

Let's first start on the problem it gives to people.

Sex is a strong kind of unnecessary pleasure. It makes you hunger for it. And when you have more sex (and don't have much of a thought about it), you will want more of it, with more partners, positions, etc. When you can't satisfy your hunger, you will become frustrated. When you can satisfy your hunger, it will reappear soon after, stronger.

Meaning that, unlike what modern mainstream media opinion says, having casual sex makes you unhappier.

When you add that to modern mainstream media culture, that says that sexual pleasure is super important to a healthy life, we get a disaster for society, as well. Why? First because of the effect this belief has on people. Those that don't find partners end up feeling like crap. Others will look for partners that satisfy them sexually. Meaning that they will select marriage partners based on physical pleasure, instead of more important traits.

And then, you have the second problem. The effect of the combination of this belief and promiscuity on marriage. Someone that think pleasure is super important, that is addicted to it, will think that marriage is about sex. What is the problem with it? Eventually romantic love cools. With this, we end up with adulteries and broken relationships.

And what is the problem with broken marriages? The effect it has on kids. Couple the problems of being raised by divorced parents with the belief that pleasure and romantic love are the most important things in life and you end up with a broken generation.

Because prior to the middle of the 20th century, condoms weren't really a widespread thing. Having casual sex with random people meant producing bastards and spreading venereal disease.

You're basically asking why a set of moral values that evolved in response to the way things had been for literally tens of thousands of years hasn't change in a single generation. These things take a long time.

This is bullshit.
Producing bastards and spreading venereal disease are the smallest problems.