Before we get to what they'd do, let's remind ourselves why they'd be bitterly disappointed. First off, Elly's default reaction to anything she doesn't like or understand is to assume that it's part of some screwball conspiracy her paranoid brain cooked up to explain away the fact that she sucks at every damned thing she ever tried to do. It's not that she's a lazy, inattentive, short-tempered boob without any sort of stamina, courage or brains, it's that people are out to get her. Second, we have to remember that dickless, spineless, heartless, gutless, brainless failure John is a craven, loutish weakling who craves obedience because he thinks that the give and take of normal human interaction must be wrong because it required him to give as well as take. Third, we have to remember that they have managed to convince themselves that having to feed, clothe and house defenseless children who could not survive without them makes them the victims of a band of extortionists; this is why they need to own horses so anything that stands in the way of "justice" is automatically bad.
What this tells me is that the poor fool who dared out him- or herself would be subjected to a long, loud, incoherently belligerent lecture by two assholes making that idiotic palms-out pleading gesture that makes me want to punch their stupid heads off about the horrible, selfish ingratitude of an evil deviant who wants to break their hearts and evilly and selfishly not get their money back. Said un-Patterson would thus be like Dead Lisa from Funky Winkerbean. As you may or may not know, her asshole dad's reaction to her having been date-raped was to spurn her like a radioactive leper with Tourettes because she somehow brought shame on him. Worse still, the pathetic zombie managed to convince his worthless self that he'd made things right by apologizing to her cadaver. Thus it would also surely be with John and Elly; we'd be told that they were okay people after all when they whined piteously about the tragedy of it all when they were interring their dead child's ashes.
It's either that or a death by a thousand cuts owing to having to endure passive-aggressive suggestions that just possibly, the person could be mistaken as to what he or she really is and simply needs to explore avenues that would allow Elly and John to have grandbabies produced normally. We'd also have to endure nonsense about how they don't actually hate the person's partner sight unseen merely because they stand in the way of making parenthood finally pay off as much as they think that all children date someone who isn't actually right for them because that's what children do. Said comments would be as insincere as John's attempt to bullshit April into believing that the Housening hadn't already been decided. Given John and Elly's chronic self-awareness deficit, the two of them would be baffled and angered when domineering apes were given their right names.
The reason I feel confident about assuming this has to do with how Lynn slammed Liz with the Mighty Fist Of Ham because she didn't keep herself pure for Anthony. Instead of marrying the sunken-chested drone right out of University, she had to Learn To Love Again after having her heart broken by false heroes. What's more, her running away forced Anthony to settle for Thérèse and have to endure the endless torment of having a child who isn't related to the Sainted Elly. What this means is that unless the gay kid gets magically cured by one of the idiots from 9 Chickweed Lane, his or her destiny would be to either die in the same gutter as Becky or envy her her release from a world of judgmental, self-serving assholes.