Wow, thank you so much for sharing your current situation and experience. It’s so funny, we tend to think therapists as being potentially wiser and more informed about ourselves such as on these topics, yet as you said, what your therapist said was short-sighted and simplistic. Absolutely. Anyone can leave any person, at any time. Whether monogamous or not, living together or not, married or not. There is no protection or security from this, except for in our heads. And the more we try and shoehorn our relationship into fitting outside expectations, the more this can wreck the relationship (I think).
It sounds like your current scenario works quite well for both of you. This is what matters. There is no one right way to have a relationship, very contrary to what our culture tells us. There are many, many ways. And yours is one of them :-)
Thank you for reading, and for taking the time to share. It means a lot to me. With warmest regards, Brooke