My boyfriend often says that I am fat and ugly, and then explains that I am joking.

A man who can't joke or has you in his heart will joke like this, because he thinks you will always belong to him and will never leave him. For a woman, such a joke is really not funny at all, but it is precisely because she loves your performance. If I guess correctly, your boyfriend is Capricorn. If so, you should forgive him. Capricorn doesn't understand romance at all and won't please anyone. It will only protect you and care about you with actions.