You acknowledge that heterosexuality is not implicitly mutually pleasurable either. Mammals are driven by &quot;fun&quot;, they do activities that stimulate dopamine receptors and all that stuff, they avoid or resist activities that cause pain. All one needs is that first memetic spark, the dom that treated his male sub with love and respect, to begin a mutual and strong homosexual relationship that can be passed to future generations. As men break the female monopoly on sex and begin to pleasure each other, the broader social dynamic shifts from &quot;Grug and Greg compete on the plains, gather resources, and trade them for sex with Wilma&quot; to &quot;Grug and Greg have sex, cooperate on the plains, gather resources more efficiently, and then begrudgingly knock up Wilma to continue the species&quot;.