Before more jumping to conclusions happens, could we clarify something? I think a bunch of people are reading the statement "apparently I'm a unicorn... he told me" to be that the couple called her a unicorn.
My impression was that the READING she was doing was calling her a unicorn, and what he told her was his marital status. Yes, that happened too late in the game, sure. But it wasn't him calling her a unicorn - that word was her own (and was, notably, confusing).
So, jeb, could you clarify who said what and when? Is this the correct order of events?
1. you met and were intimate
2. he told you he was married
3. you met his wife and thought she was awesome and gorgeous
4. you're confused and start reading things
5. you find out there's this confusing term unicorn
6. you come to this forum asking for advice.
Is this the right order?
I think it's worth hearing your reply before more responses come in assuming that you're being asked to date the couple, not just him. There are just too many assumptions floating through people's feedback until
(I'm not ignoring that he told you his marital status until after you were intimate - his truthiness is a separate ethical issue from the use of the word "unicorn" and could be worked out separately...)