Why do some gay people want to have kids

im confused cuz i was told that evolution made gay people to reduce population

Evolution DOES NOT WORK THAT WAY.
It does not have a will. It's merely "people with gene X pass it on more often, so gene X spreads faster through a population." Homosexuality cannot have evolved to stop overpopulation, as overpopulation has nothing much to do with passing genes on. Yes, overcrowding can lead to a collapse of civilization, but overcrowding is a rare event. Humanity is the only species to really experience it wholly, and our existence isn't really threatened by it (yet). As it takes repetitive struggles (such as disease or predators) to cause evolution, and overpopulation can only occur once (affecting an entire gene pool simultaneously), overpopulation cannot be a method of evolution. Also, as genetic transfer is the sole means of evolution taking form on humanity, and gay people by nature do not have children, it does not make sense that a trait would evolve that takes one out of the gene pool.

It makes far more sense that, as society has developed, people have developed notions of sexuality that were not previously there. Back in biblical times, there were no such things as homosexuals. Not because no one was attracted to the same sex, but because society had never thought of it, so it wasn't an option. After a few thousand years, we start forming concrete ideas of "straight" and "gay," instead of the fluid notions that were previously there. Basically, being gay did not exist a thousand years ago. People may have been, by our definition, homosexual, but they could have relations with the opposite sex easily, as there was no notion that they were any different from "straight" people.

Probably because there's two different forms of human emotion expressed between wanting kids and who/what you're sexually attracted too.