James Bond does not get Gala Brand because she stays with the man to whom she is engaged. Gala would have been just another affair.
Dominic Flandry does not get Kathryn McCormac because she stays with the man to whom she is married and he does not get Kosarra Vymezal because she is murdered. Both relationships, if continued, would have led to marriage. Djana's curse (see here) might be relevant.
Will SM Stirling's Rudi Mackenzie get Mathilda Arminger? I do not know yet. If he does, it will be a lasting marriage/handfast. While speaking to the Wanderer, Rudi sees himself with Matti and a newborn baby but how trustworthy are the Wanderer's visions?