Should I mention my relationship status in my interview thank-you letter?

I just had an interview for Part-time Administrative Assistant potentially evolving into a full-time position with the VP of Operations and the boss of a worldwide company. This is my second-round of the interview process. While my work history is in good standing, I felt that my personal relationship status did not sit well with the boss. I was asked where I lived and if I was renting a place. I told him I am living with my boyfriend’s family. This raised concern. (1.) On paper it seems like I’ve only been with my boyfriend for 1 year which doesn’t look too stable, but I’ve known him for 6 years and been in a relationship with him for 3 years, so we’re pretty stable. I made sure I emphasized, “We’re stable.” (2.) I’ve only been living in the area for a year, which also doesn’t look too stable.

I still feel uneasy with how the interview went after he asked me about my personal relationship. They’re looking to hire someone committed and willing to grow within the company. How can I assure them without demeaning the boss and the VP of Op. that I am stable in my relationship and my location? Should I even mention it all in thank-you interview letter?