Interest in sex does not decrease as we age, even when a person has dementia. In some cases, her actions will be interpreted as sexual when she is trying to communicate. When behaviour expresses a sexual desire, give her privacy. She is an adult and has the right to be left alone.
Sexual behaviour involving a couple (when one or both has dementia) is a difficult subject, especially if not with a spouse. While it may be tempting to try to stop the relationship, keep in mind he likely no longer remembers his partner or even being married. Allowing it to continue may hurt his spouse and family, but further responsive behaviour may occur if he is either prevented from seeing his new companion or made to feel guilty. Distraction can work to disengage the behaviour, at least for a period of time.
Please keep in mind that this refers to consensual behaviour. If the sexual behaviour is not consensual (e.g. resident to resident or resident to staff member), use the strategies below.