Okaa-san Online Dropped:

Pushing someone's Mother as a sex object is disgusting.  It implicitly endorses adultery, ie, cuckoldry, as well as the absurdity of older women being attractive which just isn't true.  The incest is icky too but it needn't even be invoked.  If it were an unrelated mother, some 50 year old married cougar, suddenly jumping in the bath with you and asking to clean your ears, it would already be hell.