Suddenly my Father (92) with dementia has become interested in sex. Any advice?

Setting boundaries may not be effective since your dad has dementia but you can try to admonish him when he begins to talk about sex. It might work.

Also, try to redirect him. When he starts in on the subject of sex have some other topics you can redirect him with. Pretend you didn't even hear him and get him talking about something else: "Dad, did you see that squirrel out there? We have so many squirrels, do you think I should set traps and release them in the park?" Or, "Dad, remember that vacation we all took to ________? I was thinking about that the other day and remembering how fun it was." Another good redirection tool is history. Our parents lived through some amazing and tragic history. You might want to try to get your dad talking about something historical. For example, the Kennedy assassination or Vietnam. I'm sure your dad remembers a time when there was no tv. What was that like? Did he and his family gather around the radio every evening?

Get creative. Some topics will work to redirect him and some won't. Redirecting is challenging and exhausting but effective.

What is it about 90 year olds with dementia that wake up one morning and think they are a lothario!?!?!
My father is a crusty, nasty, ill tempered creature, yet he now thinks he is a gift to women. He usually waits for an audience and the regales all with a recount of his dating adventures from the 1940's.
I am afraid you have a problem! You need to set some firm boundaries and reinforce them often. Please tell me you have no daughters, granddaughters or other young women in the household. You must inform them of this new disturbing development and take measure to protect the rest of your family.
Hang in there...remember this is none of your fault.