Complex sentence: The classical rabbis instructed that masters could never marry female slaves--they would have to be manumitted first; "Gittin", 40a similarly, they ruled that male slaves could not be allowed to marry Jewish women.
Split into simpler sentences: The classical rabbis instructed that masters could never marry female slaves. They would have to be manumitted first; "Gittin", 40a similarly, male slaves could not be allowed to marry Jewish women.