Nice question. I'd say yes, it is a sin to God. being gay or being a lesbian defies the entire purpose of our creation. God created Adam and Eve. Not Adam and Steve. Not Adele and Eve. Adam and Eve. He said go into the world and multiply. That's not something that two men or women can do. It's something that needs to be done with both parties present.
Also setting aside the original purpose of mankind on Earth, we can also see in the book of Genesis, God literally rained hell on a city because it was filled to the brim with homosexuals. I think his opinion on the matter was made perfectly clear with that.
Personally I'm not a fan of homosexuality but I also believe that a sin is a sin. None are bigger than others. So the same way God despises homosexuality, he probably also despises fornication, theft or even white lies.
The answer is depend on who are you. Are you a religious person that believe for what is the Lord told you, or you are un-religious person that "F this S, i'm going forward". For the first type, and that's including me. that's a big sin. It's really feel's like you mocking God for doing such a stuff. God has been wise for creating Man and Woman, Adam and Ever, not Adam and Gregg. * sorry gregg. And it's a real pain in the ass inside the society. Some people really strict about this question, because they having a BIG respect of Human Right, but then can you facing the big result if you let such thing's going well in the society?
Let's just saying, you let them be. without ask them or even convince them to change to be a straight one. To be a better one. Even when they said "but it's hard" but then you still be there for them as long as they willingly to change. Instead of letting they do whatever they want to do. If this gay or lesbian thing's going normal, spreading, and ended up like being a lifestyle. it'll affect many sector. Have you forget what happend on Gomorrah? no, people remember it very clearly, but instead of change to be better, they just keep replay the same episode over and over again.
I'm not telling we should avoided gay people or lesbian, hit them wherever you meet them, humiliate them, or bullied them, no, a big NO. But ask them to change, convice them to change, be with them when they in the middle of it. It's better.
What I think matters very little here, we instead have to explore what God's word says about that. Let's explore a couple of scripture.
Leveticus 20 vs 13: If a man has sexual relations with a man as one does with a woman, both of them have done what is detestable. They are to be put to death; their blood will be on their own heads.
1 Corinthians 6:9–10: 9. Or do you not know that wrongdoers will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor men who have sex with men 10. nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God.
God's word is clear on that.
Again? That question seems to pop up ever so often. It really seems to bother certain people. Well there seem to be no other problems on our humble little planet. But as often as this question will be asked. My answer is and remains NO. No sin here....Next question.
Have a good day :)