Asalaamualaikum wa rahmatullahi wa barakaatuh,
Ok, before anyone reads this, i'd like to point out that everything i say will be based off of whatever little knowledge i have of Islam and definitely shouldn't be taken as anything scholarly.
First, i think we should clarify what is meant in Islam when we say "homosexuality is a major sin." From what i understand, when any islamic writing speaks about the the SIN of homosexuality, then it is clearly ONLY speaking about the ACT of homosexuality and not the feeling or inclination towards homosexuality. I mean, i think it is easy to even deduce this logically because let's say for instance, a man is walking in a super market, and he sees a chocolate bar. Suddenly, he gets this inclination to take it and put it in his pocket, or in other words, steal it, so he can eat it later on. However, the man decides to fight the inclination because he knows it's wrong, and he walks away without it. Did he sin just because he FELT like stealing? No. Sins are not committed through inclinations or feelings, they're committed through following through with these illicit inclinations and manifesting them through actions. So, if that same man had let himself fall prey to his desire to steal and actually stole the chocolare bar, then he would have commited a sin.
In the same manner, the way i've understood homosexuality in regards to Islam is that the ACT of homosexuality, such as, homosexual intercourse, or being involved with the same sex, is Haram and not the inclination towards homosexuality or solely having feelings for the same sex. For those who claim that having the feelings of homosexuality are haram, then they are claiming that having any bad inclinations, like to steal or to fornicate or to hurt someone etc. are all sins as well, even though none of them are committed. However,I don't believe that is an islamic teaching.
The Prophet Muhammad(s.a.w) said, "Allah has recorded the good deeds and the evil deeds. Then he made them clear: He who has intended a good deed and does not do it, Allah writes it down with Him as a performed good deed; but if he intends it and does perform it; Allah records for him from ten good deeds to seven hundred times that or more than that. If he intends to do an evil deed and does not do it, then Allah writes it down as a good deed; but if he intends an evil deed and performs it, Allah records it as one evil deed" (Sahih Al-Jami)
True believers, even if they have feelings for the same sex, would fight these feelings knowing that they are from shaytaan. Believers would make dua to Allah to help them overcome this fitnah. In the end, if a Muslim finds him or herself having these kinds of feelings, they should see it as a test and through prayer and struggle, get through it by the will of Allah!
I think many times what happens when we have this kind of discussion is that we all use the same word, for instance, "Homosexual," but we mean different things. One person may say "homosexual" and define it as anyone who has feelings for the same sex, while another person may say "homosexual" and define it as one who has feelings for the same sex AND also acts upon those feelings by having sexual intercourse or relationships with the same sex. I believe Islamically, the first person is just a regular Muslim who is being put through a test by Allah, and is hopefully going to be successful by fighting against their lower desires(nafs), while the second person is a Faasiq(Flagrant sinner) who has succumbed to his lower desires(nafs).
Last point, many people today(hopefully not Muslims) like to justify the act of homosexuality by claiming that it is caused genetically rather than through environmental factors. First, it is not proven to be genetic. Second, to a muslim, this is beyond the point. The point is, whether Homosexuality is genetic or environmental, it is Haram to act upon it. if it is genetic, then the true Muslim will fight against the feelings, and if it is environmental, then again, the true Muslim will fight against the feelings. In the end, the believers are always successful!!
I hope that helped, insha'Allah! Asalaamualaikum!