A couple where both can produce sperm is unlikely to produce offspring either. Still don’t see what that has to do with anything.

I think the point is that the pregnancy risk issues you pointed out do not apply to lesbians, therefore do not help explain the lower promiscuity of lesbians compared to gay men.

But it does, if you consider the evolutionary perspective. Biologically, women are far more careful about sex, for the reasons I described. Sexual orientation is not hereditary by definition, so the evolutionary imperative and cost benefit analysis both stand. This is also why women prefer older and wealthier men.