That's a headline. What are you picturing? I asked Meade, and he had the same misimpression I had: So Friedrich Engels was gay.
What portal of cultural change did we just pass through that, upon hearing a man had a lover, we pictured another man? It came more readily to mind that a man had a male lover than that a woman was involved in the Marx-Engels intellectual project.
But the article — at Smithsonian — doesn't portray the "radical lover" as much intellectual help. Slogging through it, I see that Mary Burns's help came in the form of house and sex work. (Engels didn't believe in marriage.) The article pushes us to perceive intellectual contribution in her connecting him to the lives of those in extreme poverty.
Had the headline suggested that kind of help, we'd surely have pictured a woman.
As it is, the headline's "father" metaphor invites us to picture sexual interaction that produces a pregnancy. Within that metaphor, Engels ought to be the female partner, since he's the one who goes through the equivalent of pregnancy and childbirth. The intellectual work product is the baby. Even though pregnancy requires man-woman sex, if we visualize Engels as the pregnant one, we expect a male partner.
I got to that article via Joe Malchow at Power Line, who gives his post a sharper headline: "Friedrich Engels Was an Entitled Jerk."