I have thought the same thing. It never made sense to me. But religion sure as hell made me feel guilty about it for as long as I can remember. I don't think god is against it at all. But maybe he or she or it understands that it could get you into trouble.
People use what is good and natural to gain power and control. Many cults/cultists use the control of sex to exercise control over others.
Sex is powerful: powerfully motivating and appealing, like fire. Also subject to powerful misuse. That's why God puts limits on it, just as you limit (control, contain) fire.
If people didn't enjoy sex, would they bother to procreate?
Stroke of genius. By someone. Or something.
God didn't forbade people have sex or procreate. He encouraged it. People were to "multiply and replenish the earth." And so it has been since the beginning. Only too well!
I find it telling that almost every cult leader, present and past, has received revelation from god to have sex with many of their female disciples and often the wives of cult followers.
Makes you wonder how god communicates those revelations; is it to the upper or the lower brain?
I no longer believe the evil church leaders (or god) are the driving force behind such beliefs.
They are simply providing what the followers want.
And we generally want sex to be considered obscene.
If you can limit the options of your significant other and gain monopoly control over their sexuality, enforced by the church, you gain power and get their resources or their reproductive potential.
So a lot of power struggles and jealousy, etc.
Then if partner strays, you can report it to bishop and get disciplinary action and shaming, or get a divorce and get money, etc.
And if sex is exchanged for money and resources, scarcity increases the value of sex, so want to limit it and attach shame to it. And many don't like sex that much, so don't want to have to offer as much for the same money. Basic market forces.
So, yes, generally sex is to be considered dirty and obscene and nasty and porn is smut and filth.
Can't have people having sex on TV because supposedly it will traumatize the children, but you can have endless blood and violence.
If "Satan" (self-centeredness) didn't actually create the orgasm (and no mention of sex before the Fall), then "he" certainly knows how to co-opt it. (One might say the same "Why would" thing about opium...).
Same reason God created money and then demands 10% of it back. Basic Divine Incompetence. In fact, OmniIncompetence. I guess that is the downside of being the best at everything. :)
In the Bible, God wanted it to be for multiplying and replenishing the earth and all that jazz. I just don't recall him ever stating that having sex for fun is a sin - it's been awhile since I read the Bible anyway.
Not that we need more humans anyway. It's not like we're a dieing species.