Person A thinks that sex is a wonderful experience that promotes intimacy with their partner.
Person B thinks that sex is ok, but is just for procreation.
This is gonna cause problems if Person A starts feeling that Person B is ignoring or cutting out something they feel is important to their relationship. Person B meanwhile might start feeling that Person A is putting too much emphasis on sex.
Or consider the situation where A-ko feels that sex should be a romantic, intimate act with ONE partner and hooks up with B-ko who is all about sex as a physical pleasure that can be experienced with multiple partners.
All of them are in the right, but crossing paths like that can lead to a LOT of trouble down the road.