Mom fell a few months ago and has many memory issues. I'm her sole caregiver and we are managing mostly. She knew I am gay and EVENTUALLY accepted it, but now she doesn't remember and keeps asking when am I going to find a woman to take care of me. I shrug it off and make a joke. Thoughts? Should I come out again and cause her the additional stress?
If she has dementia and her ability to "learn" a "new" thought is impaired, it might be easiest just to shrug it off and make a joke. Or perhaps reassure her that you will find a suitable mate some day and you will take care of each other (if you think that is true). Tell her what a good model she and your dad gave you (if that is true). She knows she won't be around forever and she wants to think you won't be alone. Give her as much positive feedback about that as you honestly can.
This approach would be easiest on her.
But it is also OK to consider your own needs. I personally think that there is a difference of whether Mom is 69 or 93. How much longer are you likely to be her caregiver and live with her? How much longer can you live with this lack of recognition of part of who you are? That is a legitimate factor here. You are mentally healthy and she is not. Presumably you are the one who can absorb this kind of stress. But do consider your own needs. Also consider some therapy to support you through this caregiving journey. Many of us benefit from that!
And if Mom doesn't have cognitive impairment and could actually accept and remember this fact of your life, that is different than if you'd have to come out to her again and again because she can't remember it. That doesn't sound pleasant for either of you.
Take comfort that your mother did ultimately accept you for who you are. That she is impaired now does not change that fact.
This is a good site for caregiver support. You'll encounter a variety of opinions here and that can help clarify your own thinking. Hang around!