Just because my curiousity has been aroused, I have done some research into medieval and dark ages courting customs:
And from what I've found, Garstan's concerns seem appropriate. While it is true, LMP, that often marriage was postponed until fertility was assured, this was a rather formal process in and of itself. After all, there is much difference between a modest lass seeking marriage and a lightskirt. I have read (didn't save the link. Oops.) of a 'handfasting' system, where the young couple are declared betrothed in church, with no date set for the wedding beyond a year and a day, or some such significant interval. If the couple find the fertility of the bride to be a concern, at that point they are officially betrothed, and there is no taboo upon "alone time"
(If this involves physical intimacy, it is quietly winked upon by their elders, but never truly acknowledged. The important thing is that it is no longer improper for the two to spend time alone without a chaperone, whereas it otherwise would be, no matter the rank of the couple.) The wedding date can then be set whenever it needs to be, and if the proscribed interval passes, the two are both then officially single unless they renew the betrothal.
Does that make sense or have I talked mostly in circles?