When it comes to biblical doctrines or questions, personally, my opinion doesn’t matter but what the Bible says.
Let us take a look at what the Bible says.
For this reason God gave them up to dishonorable passions. For their women exchanged natural relations for those that are contrary to nature;
and the men likewise gave up natural relations with women and were consumed with passion for one another, men committing shameless acts with men and receiving in themselves the due penalty for their error.
Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality,
nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God.
And such were some of you. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God.
I Corinthians 6:9-10 ESV
This is such a difficult question because it is treading on social equality and prejudice. But since you asked for a personal opinion, I would have to say I believe it is a sin, as much as lying is a sin. A lie is a denial of truth, and it offends God because Jesus is Truth. When I have all the physical attributes that say I’m male and I protest and say I am female, it is not the truth and it offends God.