I think it’s always a little problematic to make claims about a period of time from which no records remain; we tend to impose our own assumptions and norms and prejudices (just think of Evans’ “reconstruction” at Knossos…).
From my experience there seem to be several reasons a lot of men enjoy MFM. One, it reestablishes one’s partner as “other” because there she is, getting fucked by someone else and having a fabulous time. Two, sperm competition results in harder and more urgent erections and more explosive and copious orgasms (this is all part of the sperm competition to which you allude towards the end of your essay). Three, whereas there are so many constraints that build up over the course of a monogamous relationship, the moment another male is introduced things change and there is a greater freedom resulting from all that male energy in the room.
While some men do seem to like being lessened by the presence of another (perhaps better-hung and more muscular) male, I think most men just enjoy the sheer eroticism arising from the three elements above. Speaking for myself, at least, they’ve always been the reason I’ve continued to enjoy MFM whenever it’s been an option.