Warning: first rant in months. Probably boring. Maybe incoherent, I haven't slept a lot since Alice was born.
Also, a caveat - this is something ladies do too. I just see it more often with dudes.
I used to have single/unmarried friends who were cool. Now I have kids. But I still remember hearing the same complaint over and over from random people:
-My girlfriend (boyfriend) won't go mountain biking with me.
-I took her out on this easy ride and she cried.
-He refused to ride this easy waterbar and then walked the rest of the way down.
-I had to wait forever.
Now, getting a friend into riding is hard. Mountain biking is painful and dirty and intimidating. And it's doubly hard when the friend is a significant other. But I see the same mistake over and over. Dude, why is your girlfriend riding a $100 Walmart bike when you're on your $5k carbon wondermobile? Are you really surprised he's having trouble and not having fun?
Don't take your SO on a ride on their current "mountain" bike. Don't borrow a crappy bike that doesn't fit. Go to a shop that rents nice bikes (yes, it'll cost you $75 for the day. HTFU) and get him a bike that fits decently, works decently, and has modern suspension/gears/brakes. Get a decent helmet. Don't make her ride clipless pedals, or toe clips, or get mad if he has trouble figuring out how to shift up/down. Stop as much as she wants to adjust stuff and don't get frustrated if you have to move the brake levers around 5 times.
A decent bike will mean that your SO has a shot at having fun. And you have a shot at going on a ride as a date again.