So, in this Pride Week video...
...it appears that Elton is trying to sell the thesis that Sheila had been somewhat homophobic all along, and that she was bothered by his civil partnership because it was an official acknowledgement of his gayness.
Keep in mind that their entire eight-year rift was caused, in part, by Sheila's determination to stay friends with John Reid.
So Sheila was bothered by the idea of two men having a formal ceremony...
...but not by the wild gay sex parties that she caught John Reid having while working as Elton's housekeeper (p. 110).
I have absolutely no response to this.