These are just my opinions and observations. I think JWs don't have happy marriages if they marry as JWs. If they're married coming in, that's a different story. But the odds are stacked against JWs even leading up to marriage. Their choices for a spouse are limited to whoever is in the hall or surrounding congregations. They do not have the chance to date lots of people and see what sort of person they get along with best. They don't have a chance to live with that person and see how they get along day after day, when the other person is at their best and worst. And I know many who married too young because (a) their hormones were raging and they want to try sex or (b) it was a way out of their strict parent's house. Then, after marriage, you can't bail out without somebody eventually getting DFed, unless you seperate and remain companion free and celibate.
On the other hand, the extreme pressure to stay together probably does force couples to stay together during the "settling in" period where other couples may give up. And maybe they end up working things out and may end up a stronger couple for working through the hard times together. If not, they're in for a lifetime of hell. And with all the meetings, they have to act happy with each other in public. Sometimes, this helps. Sometimes the acting happy sort of forces you to make up. I know my wife and I can be having an argument, but put it on hold in front of others and by the time the others are gone, we've forgotten the argument, put it behind us and after the cooling down, made up. And JWs like to stick their head in the sand and convince themselves there aren't problems. Some will do the whole JW thing and constantly tell themselves they are happy and to an extent, convince themselves, even if they aren't. If they believe themselves happy, does that mean they aren't? Sometimes, until you step away from the situation, you don't realize how unhappy you really were.
My parents were married before becoming JWs and I thought they had a pretty happy marriage. Sure, they had their arguments. Yeah, my dad was controlling and stubborn at times. But my mother would defend him to the end. And when she died, my dad was devastated.