As far as the Chantry goes, I'm not sure. I'm going to guess that it means that it only happens between man and wife, which is part of the definition of its irl counterpart. (The irl counterpart forbids any contraceptive measure apart from just not having sex while the woman has an ova in the right place to cause pregnancy, which might or might not also be part of the Chantry's teachings.)
Sebastian took vows, I believe.There is no sex period. Besides, the way Carver talks about it, it sounds like more than just only having sex the "proper" way.
That doesn't seem to make sense in the context of the Chantry's teachings on the purpose of marriage as Gaider describes it, but fair enough. (Edit: Though on the other hand, now that I've actually looked up Carver's dialogue it seems like Carver's aware of the discrepancy too.)