I thought it was because it’s not a good time since things are coming to a head politically and if she gets pregnant she’ll have a lot more attendants and checkups, which are more opportunities for her to be caught being sane before it’s safe for that to come out. Also since everybody thinks she has the mental capacity of a child it might not look good for her to be pregnant or something.
It's every thing, they don't know if she can get pregnant since she hasn't had her menstruation long before marrying, they don't know what poisons she was forced to consume and what the effects were. They need to be sure all poisons are out of her system and what side-effects if any they need to be wary of. IF She can get pregnant with consideration of the poison circumstances, they have to consider what pregnancy could do to her body, of she'd be healthy enough to withstand pregnancy and child birth and postpartum.
Add to all of that the political events and well it's not ideal for a pregnancy

Why is everyone against the mc getting pregnant?
Like everyone is saying mc shouldn't get pregnant but not giving any reasons