
Well, I am sorry if I bothered anyone. But, reasoning has to do with the time period that this set in. In the Middle Ages by the time you were sixteen you were in a modern sense twenty. People died earlier save for luckier ones and so, they had different age ranges child to adult ratio, the FL wants children she has forced to do anything she didn’t want too. I am just saying Dias is very modern in his way of thinking.

Yes yes but you also forgot that people were droping like flies in middle ages and what is known of teenage pregnancy is that they often unable to concieve again bcuz teenage pregnancy wracked their uterus and even died of childbirth Yes I understand that it is possible and im not bitchin bcuz what you are saying is true but the notion that you think its justifiable to have teenage pregnancy and I know its just a manga but seriously I worry about your morals( ̄∇ ̄")

This is not the middle ages. This is a story set in a fantasy world. The parameters are set by the author.
It is true that in Aruna's culture at 15 she is able to marry. But in Dias' culture people can get married at 18. He says so in the earlier chapters.
https://www.mangago.zone/read-manga/ryoumin_0_nin_start_no_henkyou_ryoushusama/nbt/hs_v-1-chapter-4/pg-1/
The couple has a cultural difference which is why he is uncomfortable with having sex and kids with someone he considers a minor. For him to ignore his cultural upbringing just because its okay in her culture would make him a sleazeball. Most people don't just up and change on things like that on the drop of a hat. Now he may change his mind over time, but to automatically be okay with it would be weird unless he would be the type of man that's okay having sex with a person considered a minor in his culture.
Furthermore their marriage is an alliance. He is not a part of her tribe now. The whole point is that he was sent away by his ruler to start a new fief. He is the ruler of his own land, he is a noble now under the authority of his ruler. So he wouldn't need to just be okay with her cultural norms anyway. If anything right now its being brought up as a running gag for comedic purposes.
He's a grown man with a good heart and morals. Of course he doesn't want to have kids with a 16 year old. LOL