Why do you think some people are homosexuals/bisexuals?

[SxyxS]

By modifying the DNA of fruitflies(just one gene)turning them into albinos,the same gene also turned them into homosexuals.
all albino fruitflies were also gay.

therefore it is mainly a genetic thing.

As the world is not just black or white(eg bi sexuals) and humans are far more complex than fruit flies and genetics could be modified or mutated to intense or weaken homosexual tendencies there will never be one ultimate answer to this question.
Homosexuality also is part of conditioning as the ancient greeks have proven.
They were dominated by a gay Agenda and it was a wanted and common thing for male people having sex with other males(even alexander the great).You almost had no chance to become part of the elite without having sex with other guys.Plato escaped this mess by claiming that love is a spiritual thing(he had sex with women but this trick helped him to protect his anus)
Though male greeks were trained to have sex with other males for centuries and homosexuality was surpressed for 1400 in Islam you will find as many gay people in greece as in muslim countries(though muslims will deny this)
It does not matter if you surpress or worship gays.nothing wikl change.
The percentage of gay males is almost the same ,no matter which country or race you go to,therefore this seems to be part of our genetic code(the same with animals)and as the nature is not reducing this numbers and keep gays at a constant level it seems not be a defect(a defect would have been eliminated by nature or lowered to minimum eg0.0005% but would never remain steadily at5%)