My Dad has suddenly started talking about his sexual exploits as a young man in the Army. What do I do?

This kind of behavior is not uncommon in people with some forms of dementia. Their "filter", the part of the brain which reminds us to not always say everything we are thinking, is dying/becoming less effective. They may also begin to tell stories which did not happen but which they very much believe. Both of these can cause concern for caregivers.

It can be difficult but you could simply not react to the stories but move on to another subject. If it is a story about your mom (his wife) you might respond with something like "you really loved mom, didn't you" or "mom was great and we all miss her" or "tell me about how you and mom met". It can seem awkward but worry less about what others will think of him or you if he tells those stories to someone outside of the family. He has dementia and will engage in a lot of behaviors which will be difficult to explain - it is simply part of the disease and part of the caregiving process.