Who do you think should come first your SO or Your Children..

Ok so I have a question... Why do people think that having a great relationship with their children is more important than having a great relationship with their SO... I believe in if you take care of your relationship first it's a better affect on having healthy relationships with your children... Especially if yall are in a serious relationship, I'm not talking about 2 people who barely know one another and had sex then had a baby and have to learn to co parent but I mean yall in a serious relationship for a good amount of time yall have kids then yall relationship comes 2nd to being parents.. I don't think your relationship should ever come second it always comes first especially if yall children are not from previous situations, now previous situations may cause different feelings and that's understandable but I'm talking mainly about couples who are happy until they have kids then everything just goes downhill from there because they stop working on the relationship because they are more focused on being parents and loving their children and caring for their children more then their spouse..