I totally believe that two beautiful young heterosexuals who haven't been laid in months or years would be very attracted to each other! And since they've been on their best behavior with each other up until now, and have shared some extremely trying experiences, I also believe that their sexual attraction would deepen into something that can fairly be called "love", and which certainly would have led to marriage under more normal circumstances. I also believe their marriage would have been fairly happy if they'd been left alone, as Dany likes to order people around and Jon likes to do what he's told.
Do I believe they're soulmates? No. But then, most happily married people aren't soulmates.