Interesting discussion. I've only really come across "speculative fiction" as an umbrella term for SF, fantasy and horror, but I agree that most fantasy doesn't really focus on "what if" to the same extent that SF typically does. In a lot of fantasy, the unreal setting is just a convenient frame that allows the writer to have fun
Consider that the reason there are two genders is because DNA is a double spiral. And consider the reason we have marriage is because we have no fur.
As a biologist working for one of the world's pre-eminent genome research centres, I feel obliged to point out that you are utterly wrong. Sorry.
1. All living creatures on the planet have double-helix DNA, whether or not they have gender (and a great many don't). The double helix enables DNA to split and transform itself into two identical copies, which is vital to cell division in every living organism, but sexual differentiation uses many different mechanisms. These range from environmental (temperature of incubation in many fish and reptiles) through individual sex-determining genes (birds) to entire sex chromosomes (or sets thereof - the platypus has five pairs!!!).
Sexual reproduction evolved because mixing up the genes each generation increases fitness - an alien world with different biochemistry could easily evolve sexes without a double-helical genetic structure.
2. What about other species that mate for life, like wolves and prairie voles? We may dress it up with ritual, but we didn't invent the idea.
On the other hand, if you want to write some speculative fiction that explores either of those arguments, go for it!