Well, even if the demon didn't control them, the relationship wouldn't have gotten anywhere anyway. Sarah would have just hopeless fallen for him. And would be hopelessly falling for him at the end of this arc lol.
I'm not saying that it's impossible for people in political marriages to be happy. It's just that the chance of it being unhappy and/or unhealthy is pretty high. And I never said anything about non-political marriages will make people happy. Marriage is a big thing after all. A huge commitment...
What did they expect? The was training to be a hero first. Just because he gained the power to become the demon king doesn't mean he will immediately want to become one.
I think they meant by changing in the locker rooms. Probably a shared shower as well, so Privaty's colleagues would notice that she is a Nikke if she doesn't have nipples.