I'm certain this has already been mentioned, on account of it's obvious, but if being gay was the norm, the way the world would be drastically different as to be unimaginable. You know, on account of homosexuality doesn't result in procreation, and artificial insemination is a fairly modern invention. So, last I checked, roughly 10% of the population was gay, so if things were reversed, 10% of the population being willing to breed wouldn't result in a very diverse gene pool, leading to a lot more diseases and birth defects, not to mention probably quite a bit of inbreeding. And whereas the people who are gay in the real world frequently have to try and hide their sexuality, in this fantasy world, heterosexuals would have no such problem. If anything, the world would be even MORE biased against homosexuals, because the very small percentage of the population who'd be willing to engage in procreation would become that much more valuable. The entirety of civilization as we know it would have evolved completely differently. Hell, the entire species could potentially have evolved differently in such a situation. First thing that comes to my mind when I imagine a species where only a small percentage of the population is actually responsible for breeding are insects like bees and ants. Afterall, in order for population levels to at least stay consistent, let alone for them to grow, heterosexual couples would have to be putting out enough babies to make up for the fact that 90% of them would be gay. I.e. Heterosexual women would have to either be pregnant and pampered 100% of the time, or the species would have to adapt in such a way that birthing multiple children at once became the norm, which would require so many biological changes that we wouldn't even be considered human anymore.
I'm tired, and going in the circles. In summary, this is a really stupid question and I don't think whoever asked it actually bothered to think about it too much.