I blame it on geting old....
I blame it on getting it somewhere else. Old doesn't stop men, it might slow them down some but men are still going strong in the 70s and some in their 80s. If a man, or a woman, loses interest in their mate for whatever reason, or they are being deprived of sex at home, they will seek it out elsewhere. The marriage may be sexless but I bet at least one of the partners is getting somewhere.