The startx script uses the expr(1) command with the "match" operator.
This is a GNUism not understood by FreeBSD's /bin/sh or expr.

In particular this part causes error reports by expr:

# if no vt is specified add vtarg (which may be empty)
have_vtarg="no"
for i in $serverargs; do
    if expr match "$i" '^vt[0-9]\+$' > /dev/null; then
        have_vtarg="yes"
    fi
done


The more portable way to do this is

    if expr "x$i" : 'xvt[0-9][0-9]*$' >/dev/null; then

which also deals with
 * $i being an option like -s which otherwise also causes an error
 * not using the redundant undefined behavior of the ^ anchor
 * avoiding the non-portable \+ quantor

A similar correction should be applied to

	    # display must be the FIRST server argument
	    if [ x"$serverargs" = x ] && \
		 expr "$1" : ':[0-9][0-9]*$' > /dev/null 2>&1; then
		display="$1"
	    else
		serverargs="$serverargs $1"
	    fi

if $1 in the expr command may start with a "-".