fontspec – sans-serif as main font?

\renewcommand{\familydefault}{\sfdefault}

That is:

\documentclass{article}
\usepackage{fontspec}
\setmainfont{Latin Modern Roman}
\setsansfont{Latin Modern Sans}
\renewcommand{\familydefault}{\sfdefault}

\begin{document}
  abc
  \textrm{abc}
  \textsf{abc}
\end{document}

If it helps to keep everything clear, you can use the \setromanfont command instead of \setmainfont, although the fontspec manual doesn't mention it:

\documentclass{article}
\usepackage{fontspec}
\setromanfont{Latin Modern Roman}
\setsansfont{Latin Modern Sans}
\renewcommand{\familydefault}{\sfdefault}

\begin{document}
  abc
  \textrm{abc}
  \textsf{abc}
\end{document}