Regarding: "Wasn't the American episcopate at the time liberal?"
- The episcopacy in the USA in those days was noted for their practical support of unions, changes in laws to to assist families, and eventually, civil rights.
- However as a group, they were flummoxed by sex and sexuality whether in marriage or outside of marriage. For them, natural law, as understood in scholasticism and defined by canons and directives from the Holy See, was both a bastion against sexual chaos, and the iron bar by which they would leverage marriages, and the sacrament of matrimony, e.g., control the laity. 
- Even today, you will still find US bishops for whom this  epithet still applies: "There is no one more Roman, than an US Bishop" -  illustrative of this is Archb Francis' of Chicago term as president of the USCCB.