If you have guns and the law pointed at you and/or great social pressure to marry and reproduce, then there is little need for a natural, biological attraction between the opposite sexes.

You won’t have a population of heterosexuals, you’ll have a population prone to being submissive and dishonest (in cases where a person’s biology conflicts with what is socially acceptable).

Another idea along similar lines would be the notion that strong attraction to the opposite sex is what is genetically governed. But, like a wild animal population with traits that atrophy if not needed (e.g. the dodo, which lost its “fear instinct” and ability to fly, due to lack of predators), if another factor (e.g. social pressure) supercedes “strong attraction to the opposite sex” in terms of a reproductive strategy, then that trait might eventually atrophy and become less pronounced over the course of generations.