That doesn't really correspond to the reality of sexual behavior though. The mere presence of a partner and of a sex drive doesn't mean sex will be a desired outcome, since the traits of the partner in question are the determining factor.

Historically, a huge proportion of men have never reproduced and genetic markers imply a strong occurrence of polygyny (it is in fact still widely practiced today) In societies where status is comparatively more important and there exists a surplus of males, this is even more visible.

With basic income, selection would simply shift even more towards physical appearance and charisma than it does today.