A: You're intoxicating.
B: You're amazing.
A: Sex?
B: Yes, please. I have till five.
A: I'll get a room across the street.
{pause for lingering kiss}
B: Any health concerns we need to work around?
A: No. And I test quarterly
B: Cool. Me too. But have had HPV
A: Not a problem. You'll get condoms?
B: I'll get the condoms. Latex okay?
A: I'm allergic.
B: Ok. I'll get polyisoprene. Lube preference?
A: Anything water based. And a dam. It will may me more comfortable. And get drinks and snacks. Salty snacks.
B: Will do. Here's my number. Text me with the room number.
A: You bet! {pause for promising kiss} We're going to have so much fun!
B: Feel my heart. It's pounding. I'd better go shopping before I forget how to walk. See you soon!
The one who made you is the one who defined how you were suppose to operate.
Sin, by definition, is not operating the way you were designed or intended to.
Biblically, we know sin has consequences. Practically, we can see this even today when we see how people living outside of God's parameters will eventually result in bad consequences.
Any time you operate a designed machine in a way that it wasn't intended or designed to operate you will have consequences to varying degrees.
So the question of why something is immoral must be posed to God, who designed you, as to why this activity is harmful to you and those around you.
I can think of many ways in which this behavior would be harmful. Not all of which may be readily or immediately apparent to you. But there may be ways it is harmful which I am not even aware or can't think of, which only God has the wisdom and perception to know fully.
That is where trusting God becomes important. Like a child trusting their parent that a certain activity is harmful. Which is part of why Jesus tells us we must be like a little child when coming to God. A little child doesn't have the capacity to understand the full rammifications of what they would do. They are totally dependent on their parents to guide them about what is good or bad for them.