I have 3 gliders, all about a year old.
I keep them in two cages next to each other.
My first glider, Marble, is a neutered boy, and he is super sweet and is friends with both of the other two. He alternates between their cages every few nights.
My other boy, Barack, is not neutered (I thought he was when I adopted him but it turns out he isn't-- more on that later). He is pretty feisty and stanky and all around pretty gangster, he wou;d definitely like to mate with Lil if it were netirely up to him.
My third glider, Lily is my girl, and tbh..... she is very ferocious. Lily is definitely the most energetic of the three, and she is generally pretty vicious and interested in devouring anything that squirms. She occasionally plays a little too roughly with Marble, and I have had to seperate them mildly forcefully a few times. Marble is a big boy but e really couldn't take Lily if she decided to fight him in a serious way. (not that she has a reason to, just for a perspective on how much of a ninja she is) As for Barack, while she seems interested in him to an extent (chattering at each other, etc) but the few times they have gotten together, she has not allowed him to really touch her at all.

So my questions are 2;
1: Is it possible that Lily would not allow Barack to mate with her by preference?
or
2: is it more likely that he has knocked her up without me knowing and she just doesn't want to be a mom?

If I have learned anything with sugar gliders, it's that they have huge personalities and the do have preferences. I don't think it would be totally unheard of for her to be selective about mating especially if she is as feisty as you say. She may come around, she may just not be ready.