Antonia Fraser talks a bit about this in her very interesting book "The Weaker Vessel: A Woman's Lot in 17th-Century England". She makes the same point as you do: that Catholic women who were interested in more than just marriage, or who just didn't want to marry, could join a religious order, but Protestant women didn't have that option - it was marriage or nothing. (AF points out that while English Protestants considered religious orders a very suspect, "Popish" idea, many educated, intelligent Englishwomen might have benefitted from having that option.)