section-protegee

\documentclass{report}
\usepackage[frenchb]{babel}
\usepackage{lipsum}
\usepackage{environ,calc,needspace}
\newlength\psectiontotheight
\NewEnviron{psection}[1]{%
  \settototalheight{\psectiontotheight}{%
    \begin{minipage}{\textwidth}
      \section{#1}
      \BODY
    \end{minipage}%
  }%
  \ifdim\psectiontotheight>\textheight
  \else
  \needspace{\psectiontotheight}%
  \fi
  \addtocounter{section}{-1}%
  \section{#1}
  \BODY
}{}
\begin{document}
\chapter{}

\begin{section}{Première section}
\lipsum[1-2]
\end{section}

\begin{psection}{Deuxième section}
\lipsum[1]
\end{psection}

\begin{section}{Troisième section}
\lipsum[1-5]
\end{section}

\end{document}