These aren’t just arbitrary “social norms” though. Cost of accidentally getting pregnant and child bearing is very high and it simply does not exist for men.
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.