It's quite well known now that homosexuality is something you're born with. We even know that it's passed on down from the mother, and can be passed down even if the mother isn't gay.
Without getting too complicated, we can look at brain scans of people and see what parts of the brain is working when we see someone we find attractive. The interesting thing is that different parts of the male brain will fire when we see somone attractive, and entirely different parts of the female brain will fire when they see someone attractive. So sexual attraction works slightly differently depending upon our sex.
However, a study I read some years ago tested this with straights and gays and they found something remarkable. A gay man's brain will fire like a female's brain when he sees someone he finds attractive, and a gay female brain will fire like a male brain when she see someone attractive.
Pretty neat, eh?
Yes, we are born straight or gay. It's not something that can be learned, and it's not something that can be changed.