Meaning of the word "who says A, must say B" in English

What does "who says A, must say B" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

who says A, must say B

US /huː seɪz eɪ mʌst seɪ biː/
UK /huː seɪz eɪ mʌst seɪ biː/
"who says A, must say B" picture

Idiom

if you start something or commit to a first step, you must accept the consequences or finish what you have started

Example:
You've already invested so much in this project; who says A, must say B and see it through to the end.
I didn't want to go to the second meeting, but who says A, must say B.