As to the Metrodorus comment I would go further and even question the accuracy of the quote.
As usual with such fragments, we don't have the full context, and to say "sex never profits" we have to ask what is the original wording and what is really meant by "profit."
So far as we know the Epicureans were not in the habit of talking capitalist theory, but they were in the habit of finding pleasures to be pleasing. That sex is generally pleasing goes without saying, and we aren't in the habit of condemning pleasures unless more pain than pleasure results. It would not be accurate or consistent with Epicurus to say that "sex always produces more pain than pleasure" so far as I can tell from the overall surviving texts.
So I would not take this quotation as sufficient cause to question that sex was being carved out as an exception and was intended to be labeled as a pleasure that always produces excessive pain. It's much more likely that there is missing context, or translation issues, or even intentional slanting of the way the text has been transmitted.