Gay innuendo isn't sexual tension. Not in my opinion. It's just a bit of humor because in the 21st century people make assumptions about two bachelors of a certain age sharing a flat. Cumberbatch said it himself about how people would assume that about Holmes and Watson. They do love one another, but it's not sexual love. It's friendship. Some think it's slashy. I don't.
I didn't "theorize ahead of the facts." I expressed a subjective opinion about one particular fictional character on a fictional show. I enjoy Watson as a male character. Martin Freeman's performance is outstanding and I don't have to apologize for liking it. If you enjoy Elementary, that's great. Neither opinion is right or wrong. I didn't ask "what the hell is wrong with you?" for liking a show.
If TPTB switched one character, yes I'd rather they switch both, like when there was a revival of the Odd Couple on stage. Felix and Oscar were female this time. My parents saw it and thought it was great. Thank you for being a total buzz kill and implying there's something wrong with me for prefering an actor in a particular role. No, not all male/female interactions on screen have tension, but when only one iconic character in an iconic pairing is changed to the different gender, generally writers will fall prey to that temptation.
I'd never heard of that Shirley Holmes show. I think the Joanne Woodward vehicle sounds more promising.