It's true that sometimes I lose sight of the fact that moicheia and 'adultery' do not quite amount to the same thing.
Relevant to the question of how moicheia, for Aristotle, violates the doctrine of the mean, are the following remarks which Lesley Brown has kindly sent. I post with her permission:
Adultery is used - understandably - to translate Aristotle’s moicheia. But we should note the following.
1. Aristotle says the very name implies badness; i.e. it names, in effect, ( a subset of) illicit sex. That is the sole and simple reason why (for Aristotle) it does not admit of a mean. Compare Rhetoric 1174a1ff; if you wish to justify/excuse your conduct, you deny that your intercourse constitued moicheia. (Likewise you may admit you hit someone, but not that it was hubris, same passage.)
2. Moicheia is typically committed by seducing a free woman, usually but not necessarily a married woman. (See Dover, Greek Popular Morality p 209; D.MacDowell, The Law in Classical Athens, p114)
3. The moichos/adulterer offends against the free woman’s husband/head of her oikos. Being inconsiderate to one’s wife is not what makes it bad. The moichos may not be married, and for a married man to have sex with a prostitute or slave is not moicheia.