Sex for reproduction, I guess God goofed by adding the [bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse][bless and do not curse] to that.
For men and women to even want to mate, there must be a strong force to drive them to each other. That force, created by God is puberty. At puberty, not of your own will you start having sexual thoughts and fantasy. This happens automatically. This is not lust or sin. Sexual desire comes from God. God created them. Having them is not sinful, unless it brings harm.
People will always confuse lust to be sexual desire, it is not. Lust is not even a sexual word. It is an act of greed that can exist in many factors, lust for power, wealth, ect. Sexual lust is the desire to take something that doesn't belong to you or habits that would cause harm.
MB does not harm. I guess if you let it control to the point you couldn't function, then it would be sin.
MB starts at age 2-3, then comes puberty and sexual thought is added to it. It is part of the natural force that helps drive us to a partner. Without it, men and women would not seek each other out.
God created puberty which makes us have sexual thought and desire, then so many want to call it sin.
If that is the case, then God created sin.