This is what cannot be proven either way. You cannot prove that the urge to have sex with Joe rather than Jane is not primal, whatever the sex of the other person. Neither can those who believe that sexuality is something people are born with prove their position definitely, but at least our side has a few more things to support that it is something that a person is born with through studies and even personal experience.
Most of that argument gets into the bigger nurture vs. nature personality argument that has been going on for a long time. Science has mainly set that it is a combination of both, but still argue about how much and which has the bigger influence.
Nature vs Nurture - How Heredity and Environment Shape Who We Are
But, since homosexuality isn't something that is harmful, especially not when the practice is done by such a small percentage of the population that it does not negatively affect natural birth rate to a point where the human population would likely become extinct if homosexuality were practiced exclusively in increasing numbers, then it shouldn't matter whether it is because of nature, nurture, or a combination, nor should it even matter if it is a conscience choice.