We always use our seat belts in every vehicle that we own. That said, my DW is an adult, and depending on road conditions, she will make a decision to unbuckle and use the rest room or grab something to eat or drink while I continue down the highway.
After over 50 years of marriage, and riding with me in the various vehicles and airplanes that I have owned, she feels confident enough in my ability to not do anything stupid. Naturally we cannot always avoid the other guy but she also will not unbuckle if there is a lot of traffic in the vicinity.
The only time we have had passengers in our coach is for very short local trips and we have seat belts for all to use.