Fiancee and her friend made a deal to marry each other! Advice Needed?

I am 29 & engaged to my fiancee who i've been dating for 5yrs engaged for 2 and to be honest we fight about things we don't agree on a lot. She is great friends this certain guy who is 4yrs older than her and he is in a touring band, they've 16 and she wrote a song with him back then which he still plays it & dedicates it to her. There both really good friends and are always hysterically laughing with our friends. He told me that he & my fiancee made a packed that if neither of them were married but the time shes 40 they'll marry and have kids together, he also said he would always love her no matter what & he tells the 'cute' stories about her when he'd bring her on tour with him saying she was his good luck charm. I get on really well with him and my fiance basically thinks he is awesome, my ex is a total 'rock star' -personality, charisma and creativity, She is very like him with all that stuff, she has a talent for writing. I prefer nights that aren't a big deal were as she loves being out seeing bands and adores old school rock'n'roll. When he mentioned the packed it really pissed me off and now i cant get it out of my head. Any advice, i know it sounds stupid but its really bugging me.