So what does sin have to do with being a homo?
Sin in the Bible is understood to be anything that falls shorts of the Standards of God.
In Christian theology sin is described in two different ways. One as sin - and one as sins. In other words there is both a generality to sin - and a specificity.
Generally, every human is born into the estate of sin. Ever since the fall of Adam and Eve everyone has inherited sin covenantally as soon as they are conceived. The only exception to this may be Jesus who was miraculously born from the virgin Mary after God breathed into her - in a similar fashion to the way he breathed in Adam to give him life. In this sense Jesus became a new thing. Just like Adam was a new thing initially.
The estate of sin generally means that every human is born awaiting death. Everyone includes the pope and every wonderful delightful and good human. It also includes all the evil and all those somewhere between. This estate of sin - essentially means that humans decide for themselves what is right and what is wrong - and the core of what sin is. People are born to die. This is the estate of sin and death. A decision made by our first and greatest human and we live with the consequences that he knew would be brought about by his decision. This decision of course amounted to treason. Why? Because only God is truly able to determine what right and wrong is - and the best decisions humans make - always fall short of this mark.
Specifically, sins are the individual sins that humans make. The 10 commandments provide a basic list of these sins and every other sin is derived from these 10. The 10 themselves can be succinctly reduced to two - love God and love others. When we fail to love God and when we fail to love others - we sin. The bible lists lots of sins - some of them specifically for the nation of Israel. Some of them specifically for certain tribes within Israel. Some of them for the nations of the world and people within it. There were some sins that only Israelites could commit. And did not refer to the Gentiles. For instance it was not sinful for a Gentile to eat pork whereas for a Jew it was sinful. Of course Gentiles were considered unclean. But unclean is not to ALWAYS be equated with sinfulness. Although at times it was equated with it.
Homosexuality in the OT and in the NT is considered sinful. The point of its sinfulness is that it breaches the 7th Commandment. The 7th Commandment's positive assertion was to promote the sanctity of marriage and the making and raising of children. Homosexuality by virtue of simple math cannot create children. It requires either adoption or a surrogate. Neither of which is the ideal situation.
Similarly, we can see why prostitution is sinful. Why we see fornication is sinful. Why we see adultery is sinful. Why we see pedophilia is sinful. Why we see bestiality is sinful. Why we see masturbation is sinful. Why we see bigamy is sinful and polygamy too. This is why even a de-facto relationship is considered sinful in the eyes of God. And pornography. And incest. Homosexuality is but one of many sinful behaviors according to the bible. But heterosexual activity outside of marriage is just as sinful. The bible is not homophobic - it is pro marriage and pro children.
Unfortunately many Christians spend far too much time focused on homosexuality giving a distorted view of the bible.
And your totally wrong about nature because it is natural to be homosexual and always has been.
It is once sense natural since the fall of man for people to sin. Homosexuality is therefore probably part of the natural order of sinfulness.
We ought not compare ourselves to animals. Animals for instance might in some species have same sex attraction. Yet animals also have lots of other natural tendencies which humans do not share and would not wish to put on humans. For instance cannibalism. Would we eat other and consider that normal just because some animal species do that? Of course not. Some species kill for the sake of killing, for pleasure. Should we consider this therefore normal for humans? Of course not. Some animals eat their own facies. Should we consider that normal or natural and start doing this? No of course not. Some animals rip each other off. Some steal other species children or eggs. Some poo where they eat. Some wear clothes and some don't. The point is - humans are humans. We are not animals. We have our own rules and customs - that are quite distinct from every animal on this planet. We would be stupid to look at an animal and say - well that is normal for them - or even a hundred different types of species - it is therefore normal for us. It is a silly argument - and one that simpletons rely upon.
We choose who we are attracted too and we choose who we are not attracted too. We have the ability of self -control. We don't look at that person and just give in to our desires and lusts - there has to be reciprocal consent for it to take place. Animals just do what they want - no matter whether there is consent or not. Rapists do the same thing.
I can choose not to look at or desire any other person save for my wife. I can control my self. Can you? Or do you just follow your groin - and let it lead you where ever it wants to go?