He's not really joking about it at all. He's trying to be real about it. To show her that he can try. He's willing to try, because she's right. Not wanting to do this because of how he feels about himself- it's wrong.

"Thanks." He hasn't even started his mac and cheese yet, he's been so nervous about this, and it might have to hold off for even longer when she mentions forehead kisses and he reaches out for her hand. "Come here."