Margaret and Mordecai will never last as a couple. He'll be that fun sweet guy she really cared about but dumped as soon as she left for college. She wants a lot out of life and Mordecai is too happy being some stiff who works for minimum wage as a park groundskeeper. Even if he worked hard to be more than that, he would only be doing it to impress her and not because he really wanted it. In time, he might become resentful. At any rate, it would totally change the kind of guy he is and Margaret would find him a charming anymore.

Rigby will get married before Mordecai will. He's kind of used to being pushed around and will get bullied into marriage by some woman who sees "potential" in him. He'll be forced into getting a "real" job and live out his days as a henpecked husband & father whose kids all think he's kind of a dope. He won't really be happy so much as content.