Maybe I misunderstood, but I thought the Sex Berry always changed your pet's gender when they ate it. I've given two to my pet and it hasn't worked yet. Is that normal?

This isn't a bug, for the berry to work you have to click on the use option, which is below the feeding option. Feeding it to a pet is just going to reduce their hunger.

....and now I feel like a blind idiot XD Thanks 😅

It's alright, we all have oversights like that.