A.,
I think it is important to let her know that "everyone has a Dad." Seriously, Dads come in all shapes and sizes, and she might have gotten one that is not a great fit, or not to your liking, but really, EVERYONE HAS A DAD.
Pre-K is a great time for figuring out which thing goes where, red round peg into red round hole, tall guy on the left, woman on the right, kids in front with dog and cat. If your picture looks different, or you manage to get the round peg in the square hole, you will have to explain yourself and why you are different. (Welcome to pre-K).
While you are at it, you might want to let her know that some kids have a mom, some have 2 moms, some have only a dad, and some have 2 dads, then there are the traditional one mom one dad kids, as well as those who have an additional addult in the house. There are plenty of these people raising kids wherever you are, and even if you happen to see a couple of women with a baby, you can alsways act out the scenario of how that baby feels to have 2 mommies with her.
If there is some way for you to have him be a part of her life in some small way, even a photograph and a bit about why you did like him, once.
I got divorced when my son was in pre K, and my daughter in K. She was never phased, but Tomy's mom said he could not play with Sam because "Sam has no dad". The reality was that Sam had a dad. In New York. But Tommy could not play because Sam's Dad did not live with us. (We did some role playing and talking about family structures, and sold the house and moved to a more urban area.)
I'm not saying you should move, but it's going to take some patience to get through this, because people are small and petty and even if they don't say it to your face, they'll be thinking **as we all know, this would not be happening if there was a father in the house**, while they do little to help your daughter understand where she is in the world. Just remain calm and let your daughter see that there is not "one right way to be a family".
my 2 cents, anyway.
S.