I suspect this has to do with perception of gender roles.
A man’s role is to get the woman pregnant. The entire role is tied up in the “agressor” concept, chasing down and seducing/persuading/whatever the woman, etc. Their main roles are to produce offspring and prevent others from making use of the same fertile ground, so to speak; keep other guys away from their woman/women.
the woman’s role is longer and more complex. It’s less about havig sex so mcuh as the entire pregnancy and nurturing process.
Thus a man who feels that he is being relegated - or might be relegated - to the opposite role, to be pursued and “penetrated” would feel his basic role is being negated. (Note that in some cultures, something I saw mentioned in a discussion of AIDS in Africa, some cultures don’t consider the man doing the penetrating as homosexual, just the one on the receiving end. Thus, a person who would seem to be threatening (by their very existance) to do something like that would be percieved as a threat.
Lesbians, by simple anatomy are incapable of “trespassing” where a man should go, (not counting fisting, I suppose) so are less threatening to men. All they possibly would doin a man’s eyes is “run interference” during the chase-and-seduce or provide and enticing opening act if he gets to watch; more of an annoyance than a threat. Of course, as mentioned above, In The Good Old Days, women really did not have much of a choice about the reproductive process anyway.