Continuity: When Monica is in Richard's apartment, she gets upset about how her current situation is not fair. She says that fair would have been Richard wanting to marry her when they were together, or Chandler wanting to marry her now. This is a misstatement on her part. Richard did want to marry her when they were together. The deal breaker was that he did not really want to have more children.
Note: Although she doesn't say it in so many words, I think it is understood that she meant marry her in the way that she wanted to marry him, with him wanting children.