At least one other commenter picked up on for health. It wasn't something people wrote down back in those days, (I don't have a citation) many people then considered complete sexual abstinence unhealthy. So Franklin is saying you should engage often enough to maintain health; and Franklin was no prude, his oldest son was illegitimate, and so was that son's son, William Temple, who Franklin more or less adopted, and who returned the favor by collecting and publishing Franklin's papers http://en.wikipedia.org/wiki/William_Temple_Franklin