Somebody once pointed out that "incest" was a matter of taking the idea of brotherly love one step too far. In this case, since Eve was the only woman on the planet, I'd say the three men in her life would have kept her as busy as a one-armed paper hanger.

I suspect the point of Genesis is that all "humans" are directly connected to the first human being and share a common humanity.

I also suspect that no one at the time would have appreciated an explanation based on "a diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum."