You know I keep seeing this, but it was made clear Logia users aren't automatically intangible manifestations of their element, they have to will it (Crocodile). Further Magma-man was going after Luffy, and Ace being intangible wouldn't have saved him (Luffy) so Ace had to stop the blow with his body, which he did saying "Lol, Magma beats fire" completely invalidates Ace's sacrifice to save Luffy, which is annoying.