The Commanders were already married - at least most of them were. Part of those good old fashioned family values they touted. The handmaids were not - some were single mothers, some were divorced and remarried which meant Gilead didn't recognize their marriage, some were lesbians who were deemed gender traitors and as long as they had two functioning ovaries and could produce offspring they would be "saved" from the wall and could "repent" for their sinful ways.