--- python-single-r1.eclass
+++ python-single-r1.eclass
@@ -183,7 +183,11 @@
 	local optflags=${flags_mt[@]/%/(-)?}
 	optflags+=,${flags[@]/%/(+)?}
 
-	IUSE="${flags_mt[*]} ${flags[*]}"
+	if [[ ${#impls[@]} -eq 1 ]]; then
+		IUSE="+${flags_mt[0]} +${flags[0]}"
+	else
+		IUSE="${flags_mt[*]} ${flags[*]}"
+	fi
 	PYTHON_REQUIRED_USE+=" ^^ ( ${flags[*]} )"
 	PYTHON_USEDEP=${optflags// /,}