It is possible to be with someone successfully for your entire life, there is no doubt about this as evident by the numerous old couples that I got to know while working for a doctor's office. Now as to the attraction part of your question, humans are going to see others and find their appearance appealing, however, when I am in a long-term relationship I sort of stop noticing men. I can't speak for others on this, but really I just don't notice them when I am in a happy relationship. I can see an actor in a movie or television show and think, "Wow, he is cute," but when I meet guys out in public they no longer do anything for me. I no longer have that desire to find them attractive. But, I think that people you are not happy in a relationship or do not want to be monogamous would continue to seek out people that they find physically appealing.