But how can the oneness of God be compromised by sex or anything else for that matter?
It's not.
So God gave these bodies with all their desires and then told us that many of these desires were wrong and the thoughts around these were impure. How odd that God made us? He could have made us like plants - seeds wafting on the air. Or made us procreate with parthenogenesis? No need for sex at all. But instead God makes two sexes and they - in most cases - are attracted to the other sex, this is the natural reaction they have given the bodies they have and the urges these bodies have AND THEN they get a bunch of rules to block that.
Very odd.
It's like giving your kid a bicycle and a mind that likes riding a bicycle and then saying you can only bike on that street between 3 and 5 and if you desire otherwise - have impure thoughts - you are a sinner.
Some dad.
Living beings take birth in different bodies or life forms: one can take birth in the human life form, or in an animal life form, or in a plant life form, for example.
The soul (ie. the actual living being) is not the body.
God has created the human life form for different purposes than the plant life form, for example.
The human life form is meant for reaching higher states of consciousness.
Unrestrained action on whatever desire may come up in a person's mind (be that sex desire, desire for food, possessions, etc.) impedes the progress to that higher consciousness.
If you don't want that higher consciousness, you are of course free to act as you feel your desires dictate.