I think there's some scenes where they imply it (I was skim reading to find something fast) so the only one I saw which does show that they had sex is chapter 26. Spoiler: spoiler "A night of tossing from side to side, Feng Tingye practiced what he preached and demonstrated to Xia Yuqing what it really meant for a gentleman to not go back on one’s words. The whole night was like a potsticker getting tossed and turned continuously, only when it was very late into the night, then did it finally become silent, so she was able to lie down and tearfully fall asleep."
Actually, there's one where he literally says something like "even if you scream no one will hear you, I am the husband so it is not assault" or something. Can't remember the full thing. But yeah, all rape since she was never willing. But that is where the child comes from.
lololol I can't stop laughing while reading the latest chapter Poor Xia Yuqing her yellow book was discovered by Ultra Seme Lord!