If you’re in your 20s, you can date whosever you want, and it’s just fine. I don’t think Isherwood or Slade are in their 60s, though. That’s a bit of a stretch. Slade had kids young. Also, I think Jericho is probably pan, to be technical. I don’t think “pan” was said explicitly, but he did mention that gender doesn’t matter to him.
Age is only really a part of why I find their romance really weird.
In Flashback scenes the book presents Ish as a sort of father/mentor figure towards Joey after Slade left. The fact they screwed after having a father son relationship of sorts is what really puts a bad taste in my mouth.
And Slade and Terra is something I could go on about for ages. To keep it brief the original work leaves it very open for a interpretation and it should've stayed that way. I believe it was only really "confirmed" in a secret files books years later.