My spouse and I have been together for 33 years and it still feels like dating. We have sex almost every day and it used to be twice or more a day. My spouse has an unclaimed hall pass with conditions any time they want to take it. We are not into swinging, just a hall pass.
I do not own them, and would rather know about it and it have conditions rather than forcing them to cheat to experience a fantasy. I love them, and want them to be happy. Sex is part of that. Life can be long, and variety is a spice. It is just sex. My spouse has shown no interest in it and neither have I, but the option is there.
Other than an injury or disability if the marriage is sexless there are usually other problems that will threaten the marriage. If the attraction is gone, why stay married? The idea of not wanting sex, staying married but still expecting monogamy is bizarre.