More aggadah - familiar interpretations of Genesis, for example...where Avraham takes Hagar to wife after 10 years of no children with Sarah. The man's obligation of p'ru ur'vu becomes an issue only when he has not had any children for lo those long 10 years. She can remarry too though; it might not be her issue. Also, the Gemara's cases of problematic circumstances that don't require "chazakah" to avoid. Plus, the medical issues potentially pertaining to brit milah. Also, the prayers of Rivka and Yitzchak for children, and the famous interpretation about their lineage. Plus, the rationale of suffering for the righteous - in that God wants their prayers. Also, the sterility caused by R. Huna's lectures!