While it's better than letting people starve or get food stamps, you might find a large class of young, unemployed males who have trouble getting interest from potential mates. Historically, that's always been recipe for political unrest.

And even if there's not, if basic income becomes a thing, I'll bet there is large positive status attached to someone demanding your labor. People on basic will be second-class citizens, and there will always be political pressure from the 'working class' to reduce the drain on the tax base.

Poor people in the traditional sense would be able to pursue internships without pay to gain skills because they are underpinned by UB. An option not available today. As an employer I could take on more 'apprentices' on UBI, the contract being that I give them training, give them guidance whilst they work 'for free' until a certain point it becomes worthwhile to pay them above and beyond UBI because they're worth it.

On the other hand, there are always alternative paths to attractiveness, outside of the purely financial. In some sub-cultures, you can be broke as hell, but if, say, you're really good at playing guitar, you can have your pick.

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.

I think you may want to re-think this part of your worldview, nymphomaniacs exist but they do not make up the vast majority of the women, at least, not in my experience.

Regarding long-term partnership and raising children, that's a whole different story -- just like with men.