It doesn't hurt to tell him that this conversation is inappropriate, full stop. I know that we are often told to validate and be willing to enter the reality of those with dementia but there are limits, it doesn't mean you have to accept sexual, racist, paranoid, violent or any other antisocial behaviours.
Stop him at once and tell him that is inappropriate to discuss in public. You would be surprised how much they can get this. Simply say "Dad, it is not appropriate to discuss your sex life at any time in your life with anybody in public. If you do it we will have to leave at once". Agree with cwillie. That is a full stop.
If you cannot redirect him, leave the room, turn on music, a movie on tv. Mid stage dementia? Maybe. The symptoms that go with stages are guidelines. Most likely he has symptoms across the dementia spectrum. They lose the filters of what is appropriate conversation and discussion very early sometimes.
Can you redirect the conversation? He has dementia and because of that typical "filters" are gone. It is common for someone with dementia to talk in ways they never would have before, do things they never would have before. You have to tell yourself this is not him it is the disease. Another option if you can not redirect the conversation you can, if it is safe to do so, put on headphones, earbuds, ear plugs or anything else that might help.
With the dementia I don't know if this would work, but I might say that you're pleased he and your mother had such a rewarding relationship, but it's also a very personal, private one and should be shared just between the two of them.
I suspect he's lonely, reminiscing, and this reminds him of better days, and with dementia, he probably doesn't realize that the topic is out of bounds. And he's probably missing your mother as well; this may help him relive their relationship.
He has dementia and because of that typical "filters" are gone. It is common for someone with dementia to talk in ways they never would have before, do things they never would have before. You have to tell yourself this is not him it is the disease.
Another option if you can not redirect the conversation you can, if it is safe to do so, put on headphones, earbuds, ear plugs or anything else that might help.
I suspect he's lonely, reminiscing, and this reminds him of better days, and with dementia, he probably doesn't realize that the topic is out of bounds. And he's probably missing your mother as well; this may help him relive their relationship.