Cathy Downs ... Mari Cathcart

Yes Clifton Webb was "homosexual, effete, fussy, prissy, catty & mean" (at least in his movies) and yes he plays exactly the same role as in Laura (he would play this role over and over again in his career) but that does not make him less effective in is role. On the contrary.
He has an ice-cold nonchalant completely undaunted menacing demeanor. The audience may expect the bad guy to be a physically imposing "heavy", but what you get is a prissy guy who is as dangerous as a poisonous snake.

In fact, everything about him, including his love-or better obsession- for his wife exudes a chilling coldness. Not even his overpowering passion for Mari is burning. His obsession for her amounts to her being more of a cherished art object, just like the artifacts in his gallery.

"In this movie, it’s noteworthy that they have separate bedrooms."
Actually it does not imply what you think it does. Before the Hayes code was relaxed, married couples were always shown to have separate beds in the movies.
Rich high society couples were not only usually shown to have separate beds but separate rooms. And if you've ever read old novels, rich couples did indeed have separate bedrooms in real life. It was a status symbol, it didn't mean there was no action going on.

Jessica Rabbit
"I'm not bad. I'm just drawn that way."