I really hate to say this, but is there any evidence that the late wife would not have wanted sex with her husband? I know young people tend to think us olds have no interest in woohooing or worse, that it is disgusting if we woohoo, but that is a form of ageism. Certainly, nursing homes would rather not have to deal with people in their care engaging in woohoo, as it creates difficulty for them, so certainly are biased against the practice. However, if we are going to be so hyper protective as a society as to make it the default that old people can reach the point that they won't be allowed to woohoo with those they were woohooing with, then we should be able to have documents similar to a living will detailing who we want to be able to woohoo with once we are legally unable to give consent.
I don't know US law, but in Canada, the courts repeated many times than "advance consent" or something similar has no stading in law and is completely opposed and incompatible with the idea of continuous consent. The person may remove its consent at any point during the activity. Your idea is pretty much like allowing spousal rape on senile people. We could easily run in issues in case where the person isn't recognising people anymore (it would be like being raped by a stranger for the victim).
