I just had this happen to me. My sim had done the Serial Romantic Aspiration and was doing the Soul Mate Aspiration but the get married option was greyed out. After reading the comments here I went in and had him break up with all his girl friends he had from Serial Romantic and that fixed the problem.
The get married option is no longer greyed out and he can now get married and continue with the Soul Mate Aspiration.
If it helps this was on a new install and the first game on a new computer without any other save games on it and no Mods. The sim he was trying to marry wasn't a NPC it was Candy Behr from Windenburg.
