Mountain bikes are not cruisers with knobby tires. They are purpose built for aggressive riding on narrow dirt trails and riding over and around rocks, and tree roots. And most mountain bikers stand as much or more as they sit when they ride. If that is not your thing, then mountain bikes are probably not for you, though many can and do modify mountain bikes for town and city use.

So the question for you is, do you even enjoy cycling? The reason I say this is, to get to the point where cycling is actually enjoyable takes a little commitment. It does take awhile to dial things in, even if you get the right bikes for you. Then we get to your level of fitness. There is fitness, and there is bike fitness. Are you already fit? Do you walk, hike, run, swim, do yoga or lift at a gym 3 times a week or more? If the answer is no, then you need to take it easy as you start cycling, building up endurance slowly starting with short rides of maybe 15 minutes or so, than build up to 30 minutes, 45 minutes, and eventually an hour or more. If you are already fit, you still need to get used to the particular rigors of riding a bike, and that includes hardening up your posterior, and strengthening your core muscles and lower back to support your body when riding. If after a few weeks to a month or so of regular riding, your posterior still hurts after riding, then maybe look into upgrading your saddles, buying dedicated cycling shorts, or both. And as for your wife's back pain, I would check with a doctor to make sure she doesn't have a problem that riding might aggravate.

Maybe you should try renting some bikes a couple of times. Start with a comfort hybrid like the Specialized Cross Trail, or Giant Roam. Be honest with yourself. If after renting bikes a couple of times you still hate it, maybe consider taking up something else. And if you like it, you have a base of experience as to what sort of bikes you like and which ones you don't.