Why are some betrothal options not available?

I'm playing as Rhaegar with a starting point of Robert's Rebellion. I won the war after Aerys died during it and allowed all the rebel lords to bend the knee except Robert who I sent to the Wall. I want to betroth Aegon to Sansa but when I go to "arrange betrothal" the option just isn't there. I don't understand because she's in my diplo range and I can offer other people in my court as husbands for her so why not Aegon?