I don't think the Jim Henson Company, Disney or anyone else that might have some control over the Muppets are scared about announcing Kermit and Piggy as an item. I just think that if they finally said "Okay, they are married and have four bouncing baby figs" it would kinda be the end of the story.
And yet, I feel at least, that we have had all the Piggy thinks it's on, Kermit thinks it's off jokes. We had those for the first few seasons of The Muppet Show, and then in TMM. And then the characters grew, the situations changed slightly through GMC, MTM, and the laster seasons of The Muppet Show, and they could bring humor through Kermit being shy of his involvement, and yet heart through his being sure of it.
And then they sort of back-tracked the relationship, and start on ground zero in each movie. Interesting that Gigglesock (whose name, I confess to not knowing, so giggle-sock ye are.) mentioned Shrek, because in a way that is a good model to follow.
In the first movie, Shrek and Fiona fell in love, in the second they had their first lover's tiff's and got over them and were, once again, in love. In the third, they have to deal with having a baby.
I don't think that admitting that Kermit and Piggy are married, and giving them figs would be the end of the romance, but would rather be the beginning of a whole new set of jokes, problems, arguments, heartfelt tender moments.
Imagine, Kermit onstage introducing a number...while holding trying to hold a baby, and pull a toddler off his leg.
Or, Miss Piggy onstage for an elaborate musical number, pausing to shout childcare tips offstage to her husband. "He lost his sock!"