What incredibly enough hasn’t been brought up yet, is the distinction between gender preference and genital preference. In heteronormativity they’re assumed to be the same, but in reality they’re not. This way of thinking also lends itself to the flexibility you’re describing; note the word “preference”.

Seconding Obsidian - it’s not FOSS, but the files are just markdown, nothing special, so you’re not locked in. Self hosting is real easy, you just have to Sync the files, and everything follows. I use syncthing between my laptop and phone and am having a good time with it.