Be completely unreactive and don't let it get to you. But do learn from your mistakes and don't play up your cock as wonderful and great in future, and never seek approval for it. Make it a non-issue in any relationship you're in and stay completely humble and secure regarding it. I say this because I get the feeling she's trying to hit you where she thinks you're most sensitive. If you're big, act it, don't ask about it. If you do this, I think it's very unlikely that anyone in future will go there, because you'll have shown that your cock isn't a point of contention for you.