It happened in the Garden of Eden...........sin.
The story of the Garden is one of the most important parts of the entire Bible. And the original sin was that man wanted to be independent of G-d.
In the beginning, G-d created a spirit being - Adam/Eve - man and woman. He gave flesh to the spirit being and then divided the being into man and woman. He then united the two under His covenant where the two become one again. This was all part of God's design for us. And sex was designed as a celebration of that union under that covenant.
I hear quite a few people condemning the homosexual sex act. But to God, it is no different than any other sex act outside of His covenant. Sin is sin. And all of it is a result of us choosing to be independent from G-d.
The purpose of Christ is to bring us back to the Garden in full alignment with God's design for us to be in unconditional relationship with Him just as He loves us unconditionally. G-d is not a micro-manager. He gave dominion to us. It was always part of his plan. Yet because of sin, we forfeited that dominion. Yet Christ died for us when we were still sinners so that we could get the keys back.
No matter what affliction one may have - fear, homosexual tendencies, adulterous thoughts, envy, bitterness, etc., God has given us a way to be restored back into the people in the Garden. God doesn't ever say, "Oooh, I can't believe he/she did that!" Instead, He is saying the same thing he told Adam after he ate the fruit - "Hey Adam, where are you? We have relationship here. Why did you disappear?"
Sin has it's own consequences, which is why G-d doesn't want us to sin. And while sin can be overcome through grace, the consequences may not.
That's my two cents.