Originally Posted by Tonberry
The letter is a good idea. This way it won't seem like you're asking for an answer right away, and they can talk about it.
Going to him first... honestly, I would take it pretty personally if someone told my husband he liked me before he told me. It would feel like going to my father to ask for my hand before proposing to me. I don't belong to another person, and I can make my own decisions. Don't negotiate me like that before I'm even in the loop. I understand talking to your spouse before you talk to your person of interest (so, in your wife's case for instance, I do think she should talk to him first if she likes you) but talking to your person of interest's spouse before them doesn't sit comfortably with me at all.
I understand what you're getting at but I feel that your comparison is lacking two important things.
First of all, the man about to propose to you will probably not have that deep of a connection with your dad. If your father for some reason gets upset that he wasn't asked first, it's a small loss. But in this case it's a guy who is concerned that he may ruin things with his best friend. If the best friend doesn't like the idea of those two dating, thread starter could potentially suffer a pretty big emotional hit. Naturally he wants to avoid that.
Secondly, your father really doesn't (shouldn't) have any say about who you're dating. But in many poly relationships, there are rules about who is ok to date and who isn't. It's not weird if thread starter feels that he should find out about those rules before deciding if it's even worth a go.