Although, strictly speaking, "I write codes" may not be wrong, since, at root, it refers to "I write programming codes", i.e. the codes used to convert human-language statements of intent into stuff that computers can "understand" and execute. But my reference was more to the current usage, which is "my code", not "my codes", for "my programs" or "programs written by me".
I think you're referring to commonly accepted _usage_, not "correctness", when you say "code is always singular". The phrase "I write codes" (as in, "I write programming codes for a living)" is definitely okay/correct, IMO (for the reason given in my previous comment above) - though no experienced programmer with a good command of English would use it in normal conversation, because it's not colloquial. I was referring more to a situation like if he/she is talking to a layman and happens to use such a phrase, to describe his/her work. Remember, I said "strictly speaking" - and strictly speaking (in other words, pedantically), I don't think there is anything wrong with that statement.
But overall, I get your point. For a similar yet contrasting example, consider this real statement of an ex-boss of mine: "I can tell you what code to write, but I can't write _a code_!" which was both BS and grammatically wrong, of course :-) He did not stay my boss for long ...