If we all became gay and lesbian then that would be the end of humanity. Animals aren’t gay or again extinction. I truly believe it is a mental and psychological issue. The person was exposed to some type of abuse from opposite sex and is truamatized so they turn to same sex to bury their traumas.
Variation in sexual orientation is a regular occurrence among all animals, including humans: bit.ly/same-sex-animals. Statistically, it’s at least as unlikely for all organisms of a species in any given generation to be heterosexual as it would be for them all to be all gay or lesbian, so neither is really worth considering. I encourage you to seek out sources grounded in empirical research; researchers go to great lengths to account for their own biases. When our personal beliefs aren’t grounded in the research of experts, we risk basing our decisions and actions on falsehoods, and causing harm in the process. If you’re at all uncertain about the facts, here’s a good rule of thumb: be kind.