You have a very reductive view of biology and evolution. If "being able to reproduce was all that mattered" being gay would be something that evolution would have bred out of people. Evolution would have also meant that people died as soon as they reached menopause. Except they don't. Because the survival of humanity is the survival of the collective. So people who are infertile, people who were gay, people with disabilities etc, they all can help the SPECIES and the collective humanity survive by contributing in other ways not just reproduction. Inventing a new way to farm, that is helping the survival of the species. Inventing vaccinations, medicine, new shelters etc, these are helping the survival of the species. There is SO MUCH MORE to the survival of the species than "has fertile gametes, can produce offspring".