Your partner looks at you with wanting. And something in you closes.
It is not that you do not love them. It is not that you are not attracted to them. It is that being wanted, being the object of someone's desire, triggers something in your nervous system that feels like too much. You deflect with humor. You change the subject. You pick up your phone. You suddenly remember something that needs doing. You disappear into your head and leave your body behind.
This is one of the most common and least discussed dynamics in long-term relationship. The partner who shuts down when desired is not broken. They are protecting themselves from an intensity they have not yet learned to hold.