Jesus told us that the whole of the law is summed up in the saying,
“Do to others whatever you would have them do to you.”
It is easy to see how most of the moral code is summed up in that Golden Rule. We do not cheat others, because we would not want them to cheat us. We do not hit others, because we would not want them hitting us. We do not seduce others because we would no want them seducing us. We should not lie to others, because we would not want them lying to us.
But, what about pleasuring ourselves? That would not seem to go against the Golden Rule. Private indulgence in gobbling chocolates, in touching ourselves, in putting in excessive hours of couch or TV time would not seem to go against the Golden Rule. Can they still be sinful? Yes they can.
The sinfulness of excessive self indulgence comes from the way it takes one away from being thoughtful of others. The affection I show myself leaves me with little affection for others.