I could see Wash's point, though. Mal and Zoe have been through a lot of shit together, fought alongside each other for years. That gave them a connection, an intimacy if you will, that Wash would never share with Zoe. Not in the same way. It didn't help that whenever things got tough, Mal, Zoe, and of course Jayne would go deal with it, while Wash has to stay on board because he's the pilot. Great, he sits tight while his wife and the captain go save the world again. Some might see it as a pretty stupid type of jealousy, but that's the thing about feelings; they're not always rational.