Check out these sentences:
He does indecent gestures at me ~ But why look at him? Simply ignore him.
I cannot concentrate on my studies ~ Why think of her?
He scolded me unnecessarily ~ But why talk in between? Keep mum.
In those all answers, no infinitive markers are used i.e. "Why to look"; "Why to think", and so on.
Is it okay to omit IMs while talking that way? Furthermore, if it's okay talking, is it okay writing as well?