OK, as a cis, nominally straight, person, I can see a couple of angles.
One is the 'over the top' angle in which the gay person might believe that transitioning is unnecessary. Why not just live as a gay person in your birth gender? Why create all this extra controversy just as we're getting ourselves accepted? This assumes no fundamental difference between a gay and a trans person, missing the extra nuance that a trans person can be (for instance) a straight woman but born in a mans' body, rather than a gay man.
Then there is the other idea, which is a bit more convoluted. If a gay man falls for a man , then subsequently discovers that this man was born a woman, does that in some way undermine his gay status? I mean, that's beyond weird, but given that straight men harbour similar issues regarding trans women, it's certainly possible! Gay and straight men are not necessarily so different from each other as some folk might assume.
I don't know, all this makes my brain hurt! I accept that people are as they present themselves. I quite generally dislike people anyway, so their colour, gender, sexuality and whatever makes not a scrap of difference to me.