I have dealt with a similar situation with my Mom, with the important distinction being that she did not have dementia.
At 92-93, she was still active, enjoying life, going out (including two great trips to Disney), cooking her own meals, shopping and taking care of herself, etc. She also had a strong desire to live.
Like your Mom, she developed some minor bleeding. GYN assured us many elderly people have this and live for years with it without it being too serious, but biopsy was recommended and done. It came back as Stage 1 uterine (endometrial) cancer, and hysterectomy was recommended. She wanted to have it done, and after a we sought out a couple of opinions, she did. (We were assured that people even older than that have these surgeries, as well, if appropriate.) She was on an “elderly treatment protocol” so anesthesia and pain medications were tailored to her to minimize side effects and such. She came through surgery and recovery with flying colors.
Pathology came back as Stage 2, and radiation was recommended/offered - as three doses of brachytherapy (beads, not beam, as described above in post #15). Mom decided she wanted to do it, and she did. It was fine. I took her to all her appts, etc. No side effects other than being a little tired.
I did not tell her what to do, but I supported her decision-making as long as I knew she was being realistic and understood everything. My siblings and I also had a “family meeting” to discuss the pros and cons to all of this, and ultimately we decided that it was up to her what she wanted to do since she was perfectly capable of making her own decisions, and they trusted that I was overseeing the process, etc.
Down the road she did develop metastasis, which they say doesn’t often happen with that type of cancer (from what I was told, I am not an expert in that field), but it did happen to her, as it had her sister before her, and she passed from endometrial cancer the following year. She had no regrets about her treatment, though.
I did talk to the GYN about the possibility of not having surgery and she said the bleeding would get worse, to the point that she would likely need to wear a pad, and that it could get quite messy, and Mom was adamant that she did not want that. (And I didn’t blame her.)
In the hospital we see a wide range of what people do and do not want to do at various ages and stages. It’s highly individual. Not everyone wants to “do nothing”.
It’s interesting that your parents are still sexually active. Presumably your Mom is enjoying herself in that regard, too. In that case I definitely would consider the option of IUD if it would stop the bleeding, as that will likely interfere with their activities (not to mention make a big mess on clothing and bedding, etc.). I’m not sure what radiation would do but I would want to know more (and I wouldn’t necessarily discount it right away. Medical advances have come a long way). Some might also consider hysterectomy if that was offered, which would eliminate the bleeding altogether. Generally it is an eight week recovery before a woman can resume sexual activities, if I recall. Palliative treatment can help diminish symptoms and make people more comfortable, and that’s probably how I would think about this, but it’s hard to say since I don’t know your Mom or her exact situation. She may have dementia but it sounds like she’s not withering away somewhere with no quality of life left at all.
I do wish you well with it all.
The idea of "not fighting" is so counter-cultural and it's terribly difficult to imagine letting a loved one go. I'm sure what she did, she did out of love for you and as a mom myself (as you are too), I'd have put myself through it for the sake of my kids; I'll bet she would tell you the same thing. 
