There's no way I'm going to be embarassed to turn down desserts, no matter who made them.
Over the years when I was losing weight and maintaining I've done lots of different things, and nobody cares or gets put out. My favourite thing is to bring some food myself if I know I'm going to struggle.
Be upfront. Just say you aren't eating sugar any more. Without being evangelical or making people not want to eat that dessert! If people want to ask, they'll ask you more about it.
The funniest way of saying it I've heard is telling people you are fructose intolerant, so you can't eat sugar. And people ask what are the symptoms and you say bloating, cravings, weight gain, lethargy and depression. And the people get all interested and say "I get those symptoms. Maybe I'm fructose intolerant too!"