I call my in-laws Mom and Dad. For the record, I’m 45. Before I was serious with my then boyfriend now husband, I didn’t call them anything but “you.” I would speak to them directly and not use their first names, which felt odd for me to use, but simply say something like, “Would you like more cake?” instead of, “Would you like more cake, Marcia?” Once we got serious, I asked her what she thought would be appropriate, mentioning that I would be comfortable with Mom and Dad. She was thrilled with the idea, especially as she had always wanted a daughter and we had become fairly close.
For me, it’s not disrespectful to my parents. Growing up, my friends’ parents were always mom and dad when we spent time at each others’ homes. My brothers-in-law all called my parents mom and dad. I guess it’s all what you’re used to.