I’ve only really spent time on Key West (my dad lives there).
My husband and I went by ourselves the first time just to see if it was appropriate to take our 10 year old and he thought it was fine (he’s way more conservative about that stuff than I am). Honestly, outside of Duval St. in the evenings, you don’t see much of the gay stuff. I would occasionally see a gay couple holding hands while walking down Duval and once I saw some guys embracing, but that was about it. Most of the really offensive PDAs were committed by hetero couples (with one or both of them typically drunk). There’s a few blocks of Duval that are pretty questionable though. Mostly because of the bars and the shops with the rows of bongs in the windows.
There are really cool things to see on Key West though that do avoid, mostly, Duval. Sunset Celebration at Mallory Square, Ft. Zachary Taylor, Hemingway House (with all the 6-toed cats :D), Southernmost Point, the aquarium, the museum. You can even just drive around and look at all the pretty architecture, check out the beaches, find the Bahamian village, or go on a ghost tour. There’s lots to do. Oh, almost forgot about Jim Croce’s Pirate Museum. My daughter had such a blast there. If you can though, avoid getting there from Duval. There’s a building on the corner that has had mannequins displaying all sorts of lingerie in the windows every time we visited.
There are some neat art galleries on Duval. I forget what they’re called now, but one is across from, I think, Sloppy Joe’s and the other is down the street and away from all the commotion (that one is the larger and most interesting even though they’re both run by the same artist).
There’s also a hotel on Duval that is the tallest building on Key West. You can go to the roof and see almost everything. It’s really cool.
There’s a Ripley’s museum also across from Sloppy Joe’s, but if you go during the day it should be pretty tame.
We didn’t spend much time on any of the other keys, but we did go up to Big Pine Key to see the key deer, eat at No Name Pub, and visit the flea market. No Name Pub is pretty fun with all the money stapled to the walls and ceiling. People will write their names or a message or draw (or all 3) and then they get stapled to the walls. (there are pictures on their website)