<human>: Can you condense the text into three sentences or less?

I have a good friend that I've known for a little over a year now. About 8 months ago we were drinking and talking about relationships, and he told me that when he gets married he'd like me to be one of his groomsmen, but since they weren't engaged yet it was an "unofficial asking" (his words). 

Fast-forward to last week: He and I were going over his proposal plan and he brought up who would be in the wedding party, and to my surprise I was not included. He's recently been spending time with his girlfriend's sister's boyfriend (who has only been in the picture for 4 months) and he wants to ask him instead. When I asked why, he said "because I've known him longer" (they went to the same high school for a year and used to skateboard at the same skate park; they didn't really become friends until 4 months ago). He said that he'd like me to be in the wedding, but he wants each of them to have the same number of people in their party and his girlfriend currently only has three people in mind (her best friend and her two sisters) so I would only be in if she adds another person. 

I told him I didn't want to argue my way into his wedding or cause a fight, but that I was upset because he'd unofficially asked me months ago and now he's changed his mind.He didn't seem to understand why I would be bothered, and then our conversation was cut short because he had to leave for a date. 

They just got engaged last night. I feel like I should talk to him about all this, but I don't know how much of a deal I should make about him changing his mind and if it's worth damaging a relationship that I genuinely care about because my feelings were hurt.
<bot>: Do I have a right to be upset with a friend who told me I'd be in his wedding then chose someone else?