Romans 1 says that we can learn much about life by looking at nature.  And when we look at the world of mammals, we see that male and female are both needed for reproduction.  Same with humans.  Humans need male and female parts to produce life.

Life cannot naturally be reproduced without male and female parts.  In modern times, part of this process can be replicated in a lab of the coming together of an egg and a sperm.  But no matter what, it still takes the contribution of the eggs from the female and the contribution of the sperm from a male to produce life.

By this very nature, we see that homosexuality is not God’s design.   This is reiterated in Romans 1.

Because of this, God gave them over to shameful lusts. Even their women exchanged natural sexual relations for unnatural ones.  In the same way the men also abandoned natural relations with women and were inflamed with lust for one another. Men committed shameful acts with other men, and received in themselves the due penalty for their error” (Rom 1:26-27).

I’ve talked to numbers of people who say that this was cultural and specific to a situation.  But that is not the case.  In Leviticus 18:22 and 20:13.  This was not something new.  And there is no way that one can get around the issue that relations between same sex genders is, as we see here, “unnatural.”

We have it also in Genesis where a man leaves his family to join with a woman and become one as man and wife (Gen 1 and 2).  Jesus says the same.

“Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’?  So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate” (Mt 19:4-5).

Union between a man and a woman is the way of God.  It is the way he designed sexual union.  There is no other way other than man and woman in a marital covenant.