Straight: Wanting to have sex with someone of the opposite sex
Gay: wanting to have sex with someone of the same sex.
These things both meet their definition. You cannot for example say, "I am Gay because I feel gay even though I am a man and I want to have sex with women"... because that is nonsense. The comparisons between gay and trans are just bonkers. It's not about judging the morality of the trans person, which was the issue with homophobia.
Its about simply stating you can't not feel like something that YOU ARE NOT. All other things aside including safe spaces etc, you just cannot decide to change the definition of a word for a marginalised group to such an extent that the individuals of that group no longer fit in that group.
I do not define as a woman, therefore I am not a woman now. Nothing has taken away the oppression I suffer as a woman, I just cannot say it's due to my femaleness that I suffer it. I must have chosen to identify into that oppression.