Father Mike and Aunt Zo are like the alternate reality version of Milton and Tessie—they're the even sadder version of these two. Father Mike is uptight and a prude; Aunt Zo is a boisterous drunk. The funny thing is that Tessie is uptight and a prude and Milton owns a bar. These couples might have actually been better off with one another…
But as Paula Abdul says, opposites attract, which means prudish Tessie isn't attracted to prudish Mike, despite her efforts to like the guy. "Instead of popping the cherry into his mouth as Milton would have done, Michael Antoniou always offered it to [Tessie]" (2.5.107). Tessie wants romance, not to be catered to, so while Mike is focused on his religion, Tessie is focused on Milton. In her letters to Mike she "invent[s] a more appropriate life for herself" (2.6.50), but it's all a lie. Also, Mike doesn't like movies, so maybe he's not that good a guy after all. Just saying… Anyone we can't watch Mean Girls with for the ninety-seventh time isn't someone we want to associate with, much less marry.
While she's lying, Tessie ends up inadvertently selling Mike on Aunt Zo, and he eventually ends up with her. When he does, Zo—with her larger-than-life personality—takes advantage of Mike's less-than-aggressive nature and turns him into a welcome mat. No wonder he's so desperate to get out of that relationship—he's practically covered in her footprints by the time he tries to extort money out of Milton in the book's final chapters.
We're not sure what happens to Mike and Zo after the blackmail incident that ends in Milton's death, but we can't imagine something like that strengthening their already fractured relationship, can you?