I just realized I haven't previously pointed the metaphysicians on Less Wrong to "Possible Girls," a hilarious paper about acausal romance:
The ability to causally interact with your partner is important to many aspects of happy romantic relationships, but not to all of them. It’s quite pleasant simply to know that your partner loves you and appreciates being loved by you. A loving relationship with a faraway person can enhance one’s self-esteem and turn loneliness into contentment. As a lonely philosopher, I’ve come to wonder: If [all possible worlds exist], can I have a loving relationship with someone from another possible world? ...The answer, I think, is yes.
Even if you don't read the whole thing, don't miss the final paragraph.
I don't like how every time you send or receive a love letter from her you're adding conditions. Under this framework, that means you're changing your girlfriend! I'm not sure what the point of monogamy is if every time you correspond with your mate you switch to a new one.
I think the condition from the beginning is that you're picking a unique girlfriend who knows all microphysical facts about your universe, including the content of any letters you have or will ever write.