It is absolutely fundamental to a trans identity.
A trans identity is literally "I don't think my mind is right for my body. I think my mind is not like the minds of other people of my sex I think my mind is like the minds of people of the opposite sex"
Dear God, think will you!
Being trans is literally pointing at people of the opposite sex and saying "I believe I feel like they do"!
Otherwise, and I will say this very clearly,
They
Would
Not
Be
Using
The
Word
That
Labels
The
Opposite
Sex
To
Label
How
They
Feel
Inside