I have a 질문 (well, obviously xD)....

(I apologize if what I am about to ask is ignorant and/or offensive in anyway, I am only just sincerely curious and do not intend to offend any individual here, but..)

Why are transgenders included in the gay community? From what I understand, they mainly become the opposite gender because they feel that the opposite gender was what they were meant to be born as (correct?) What would this have to do with homosexuality 또는 bisexuality? Is it because that if they were...well, let's say a man decided to become a woman but continued to prefer dating women. Would he/she (depends on how 당신 see it) be a lesbian in a way?
Is that it?
I'm just deeply confused on this and was hoping someone may understand and have answers. That's all...