Sex is not most important element ofc, but in fact game rating depends not only on sex scene. Also, romance can become a most important element for someone. That's why, even if you watch sex on a second monitor, it won't be quite the same experience. People are attracted to romance itself, relationship with character and reaction of other characters to this relationship, not sex cutscene .
Yes, but many people are pushing back against GM4HiM's suggestion that they just fade to black. You can certainly have a romance without showing the sex act.