To a native English speaker, the term "lover" here has an unnecessary sexual connotation, which is a silly thing to highlight. They are married (supposedly), and thus he is her husband.
Warren Buffet and Charley Munger were partners. So were Jobs and Woz. Or Buffet and Gates while playing Bridge many times. There's absolutely nothing romantic or sexual about the word "partner" itself, though of course partnerships exist in those realms as well as in business, sports, music, dance and countless other pursuits.
I'll take a crack -- I think America's puritanical roots are STILL in play today, and mentioning the very idea that people have sex, even married people, instantly triggers pearl clutching.
I don't think it's rational either, but it is pervasive. If I described the lunch I had with my lover in a work setting, I'd expect to get tutted or an email from HR. It would probably be nonspecific and merely say that I am making others uncomfortable without mentioning the actual problem -- because to put it into words exposes the ridiculousness.