Desmond could still clearly remember the last time Claire brought up having children. Remembering it wasn't that hard since Claire used to mention it unsubtly repeatedly. Desmond had always argued that the world as they knew it was not fit for having children. It would be best to wait until he could somehow ensure the safety of his future family.
After playing for six hours with a bunch of energetic fairies, Desmond came to accept the truth; maybe he wasn't ready to be a father. Thinking about it carefully, this made a lot of sense. At his age, he should think about other things; the same applies to Claire.
Due to how quickly Desmond had been forced to mature following the death of his parents, he never used age as a way to judge someone. That leads him to never consider the subject when Claire talks about having children.
