Great Guy, Bad Sex... stay or go?

So I've been seeing this guy & he's damn near perfect, but the sex is bad. his head is A1, but the dick does nothing for me. as great as he is, I don't think I can stay in a relationship like this. we've talked about it several times. I've tried to get over it, & even thought I had at one point, but the same feeling always comes back. would I be wrong or immature for leaving the relationship? Note: it's not a skill issue, it's the size of his dick.