Bottom line is that we may all have our opinion regarding the cause but only God knows for sure.
I think the Pastor was trying to tell us that God already gives us a clear answer on this. From the sermon:
Is homosexuality a genetic instinct in the personality of the human being? What does Jesus say in Matthew 15:18 [-20]? A verse we sometimes read and quote:
"But those things which proceed out of the mouth come forth from the heart; and they defile the man. For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies: These are the things which defile a man."
Jesus is clearly pointing to the heart, the intellect, and the invisible thought life. That is where Jesus sharply focuses his thoughts on this occasion. Jesus means that the sins of sexual immorality begin in the thought life. When I have been sitting, listening to people in the night [on the crisis hot-line], everyone without exception has begun dealing with homosexuality in their thoughts. Often it happened in their teenage years, but it also happened when they lived in an unhappy marriage -- women did not experience what they needed from a man. Therefore they sought the company of other women -- and men likewise. Consequently it was such that they began their sins of sexual immorality in their thought life. And from there it continued to spread. And without exception, they all said that the first time they engaged in this behavior, there was a barrier within them. Everyone said that. However, when they had done it only once, the barrier was broken. So we see that man, even despite his sins of sexual immorality, has a protection through this barrier. After a few encounters, it became easier every time. Now we understand what Jesus is saying about the sins of the thought life.
Romans 1:26: both men and women exchanged natural [sexual] relationships for unnatural ones. There simply must be a choice that you make. They exchange. Then you exchange away something. It is a free choice. There was no one coercing. Exchange means that you voluntarily part with something and voluntarily receive something else. Voluntarily I part with the cleanliness and receive the uncleanness. A conscious exchange, Paul says. Consciously they exchanged, Paul says. Homosexuality is something sick. A healthy and clean thought has been exchanged for a contaminated [or defiled] one. A healthy heart has been exchanged for a sick one. A healthy body has been devastated because of an exchange. Paul says in First Corinthians 6:18: "Flee fornication." Sexual immorality dishonors the body. It should not be [merely] discussed, not combated -- you should flee from it like Joseph. What did the Lord say to Lot? Flee for your life. "Flee," the Lord consequently says.
How do you see this reasoning?