“Married only once” doesn’t necessarily mean they have to have married at all. There is nothing to intrinsically prevent a married man being ordained, but the Church has grown considerably since that letter was written. The role of a priest or bishop involves a lot more. Not to mention that at the time, most people would have been converts, and most converts would have been married, so to prohibit married men from being ordained simply wouldn’t have worked.
But that passage doesn’t it make clear that it is ok for “Bishops” to marry?