Really I think you should give her some time. I'm very sorry that she said that she was disappointed in you--perhaps what she meant was that she was disappointed that you would willingly choose to do something that hurt her so much when your father did it.
Moms typically want the best for their children, even if they don't agree with their lifestyle choices. Give her some time to see that you aren't interested in anything hurtful and that your lifestyle works. If you find yourself in a quandry--like, trying to figure out how to pick one partner to go with you to her house, you can find a creative solution like opting for a hotel room and bringing both partners. I would want to expose her to both of my lovers so that she could see that you're a family, but that's up to you. My bet is that eventually she will see that no one is getting hurt and that everyone is actually happy. Even if she doesn't, she loves you, and I'm sure she will find a way to be okay.
My mother was VERY upset when she found out I was Pagan. It went against everything she believed in, and actually sounds a bit to my family like I'm crazy. (One brother literally said that I'm doing too many drugs.) But many years later, she's used to it and keeps telling random people about me. (She outed me at a family reunion in rural Kansas. Thanks Mom.) I'm just saying, moms will do a lot to be okay with their kids, but sometimes it takes a little bit to get used to it.