I knew that he was referring to me as his wife based on context, but he was also married to someone else

I mean even the term "work wife/husband" can be seen as offensive. If you're not working together and he called you his wife while being married, I'd take a big step back from him.

I have always and still do want to marry him, I'm just not sure if that's something that he still wants and I don't know how to talk to him about it. I would do anything (short of losing my family/education/career) to make things work, but I'm so scared.

Good god, you need therapy. Go no contact with this guy and move on. This guy already has a wife and is playing you. Why would you want to be with someone who has no respect for their partner and toys with other women? I mean you really think you're the only potential side-flirt?