I've played a little bit and the first sex scene is them going from kissing to having sex with each other's wives straight away and there was no reluctance from the protagonist's wife at all.
The other Kabe no Mukou games at least had some build up so I think I'll just skip through most of it and see if it gets better. The Japanese fans were complaining about the protagonist's wife being a 'bitch' and I thought that meant that she would be sex positive like a gyaru character or something but she just casually sleeps with other people so I can agree with them here.