I completely agree with you. I was super confused by that. I am not saying they need to get married in order to have children but moving in together and having some sort of solid commitment should usually happen before deciding or planning for a baby. I know this is a one shot and not part of the regular story.
They're already married and live with each other in the original serie.. this is only a sequel
They are already living together in the original series! And I guess they got married too eventually? They have a healthy relationship so I recommend you to read the prequel series ヾ(❀╹◡╹)ﾉ~
I read the original one, and seems like there was a mistranslation in it where they say "moving together"
She actually said "We're not actively attempting to get pregnant (such as fertility treatment, not working too much so I don't get stressed etc), but we just decided to not use condom"
Shouldn't you move together first before planning to have a baby? that way when the baby arrives they can have both their mum and dad around them and bound with them? also both parents need to help one another and take turns of taking care of the baby, even before that! ideally a pregnant woman should have her partner besides her to help her during pregnancy especially when she starts getting heavier...