Fancy: I’m sorry.
 
Rarity: It’s fine, darling I-
 
Fancy: No, it is not fine! I… can’t give you what you want. Even though I promised I would.
 
Rarity: There are other means.
 
Fancy: But that’s not what you wished for! I swore to bring you all the happiness you could want and yet this is the one thing I can’t give? I’m sorry your stuck with this worthless stallion.
 
Rarity: You aren’t worthless! You are the man I love more than anything. Perhaps this hurdle is impossible to overcome but I didn’t marry you because of what you could give me. I married you because I love you and I want to give you the bliss I feel just being beside you. We don’t need children to be happy together. I’m happy right now.
 
Fancy: Rarity…
 
Rarity: No matter what, love, my feelings for you won’t change. If you see yourself as half a man, then I’ll fill the remaining half. Thats a wife’s job after all.
 

 
Rarity, despite being a successful business mare, has always wanted children. The thought of holding the physical form of ones union with their beloved is something she had always dreamed of. To see them grow and and succeed as she grew with them has been her goal for a while after accomplishing her dream of opening up boutiques. Fancy, thinking that her marriage to him might hold her back (He is very nervous about getting in the way of her success despite her assurance that its fine), promised that he would bring her every happiness as long as she chose him. So when she married Fancy, they were both looking forward to having children of their own.
 
But they later learned that Fancy was sterile.
 
Fancy took it worse than Rarity, who just took it as ‘a sad state of affairs’. Fancy, on the other hand, took it as a failure as a stallion and a husband. He promised to bring joy and ease to his wife’s life (his idea of marriage is this: Husbands give wives love and happiness and wives give love and support) yet was unable to give her the one thing she would want from a marriage. It’d take many many talks before she could reassure him of his masculinity.
 
Note: This takes places some time before they adopt Crescent Moonstone.