If you're not willing to fulfill someone sexually, then you should not make a promise to them that entails not having sex with anyone else. Similarly, and relevant to this thread, if you cannot be sexually fulfilled by a single person, you should not promise to have sex with only them: that's lying to them.
Every successful marriage I've seen has featured mutual sexual obligation--and yes, that does include having sex sometimes because the other person is in need. Most marriages' problems either start with refusal in that area, or get worse when other problems do leak into the bedroom.