Actually, out there, she probably didn't know any thing about vd---wasn't something on anyone's consciousness back then, rates were so low out there, wouldn't have been a consideration even.
that open consciousness of the diseases came mid 60s when more of guys started coming home from Nam, and the hippie movement was in full swing.
Doubt very much that Alma even knew much about the pill---it didn't come to rural areas any too fast-early 60s you were just hearing about it being approved and available, and that was in the cities---and you had to find a doctor who would be sympathetic to your case, cause not all of them would prescribe it to just anyone.
ANd you would have had to find a pharamacy to fill it. Alma would not have had easy access to any of those. Or the money to afford it.
Some guys just don't want to use rubber==Ennis sounds like one of them. It was the cheapest and easiest attainable birth control there was out in rural places. But kids didn't bother him, and since they were married, he figured it didn't matter---she was the practical one though no question.
First and 2nd times I saw this---the most audience reactions of gasping out loud came not from the sex scenes, but when Alma says this to Ennis---very few of the people that were at the first screening I went to could even concieve of someone saying that to their spouse.