I thought the only reason Philip said "not yet" when Melanie asked about having children is because he knows (but she doesn't) that their marriage is in a tenuous state right now and could fall apart at any minute. Before they got married, he bought her a bracelet or necklace that had something to do with having children, adding charms for each child. Ironically, there was a scene in which he showed Chloe the jewelry and they discussed having children. Looking back now, I think it was a foreshadow.