Interesting point. The game's javascript code for erotic message selection expects that Mouth Development can reach a value of 5, giving the exact same messages for mouth level 2 as for mouth level 0 -- so there definitely is a bug, but it's not 100% clear whether this is a case of the game's developer screwing up the default mouth limit, versus another case of them screwing up the message selection itself. I hadn't thought about the implications of the "can now feel sexual pleasure" line, so when I made my bugfix patch I arbitrarily leaned toward the former possibility.