I think you can have a gender-neutral conversation with your mother that you’re not comfortable knowing about the details of her dating life. I think that’s a reasonable request
You do have to accept that your mother is actively dating, and you need to unpack why the gender of the person she is dating seems to matter so much to you.
I think your mom would benefit from enlisting the help of a family therapist to help her kids adjust to the new status quo. But you do need to accept that you do not have a way to “stop or discourage her” from dating women, seriously or otherwise.