Section should automatically start new page if the whole section does not fit on current page

There is a simple solution that does require only a single pass ...

\documentclass{article}

\newcommand\mysection{\vfil\penalty-9999\vfilneg\section}

\usepackage{lipsum}

\begin{document}
\mysection{A} \lipsum[3]
\mysection{B} \lipsum[1-2]
\mysection{C} \lipsum[5] 
\end{document}