Voluntary but immoral or not allowed

This has less to do with morals (activities between two consenting adults) than it is about economics and power.

It's just better not to mix business with sex. (There's a saying: don't sh!T where you eat)

(And for this thesis, be sure you spell it COACH not Couch. - Freudian slip??)

Btw I have just seen a talk show where a woman working for Google in NYC said she found her partner at work. It seems the society tolerates when a couple works at the same company but doesn't like - or specifically some women don't like - when a man manager sleeps with a woman subordinate. So if the couple is on the same level at work, it is okay? Millions of people have met like that and married later, right?

Yes - if they are in the same level or in a different department for most companies it is not an issue. It is the position of power vs suborinate that is the issue.

It can and has happened where a boss and employee become romantically involved - in these situations the ideal thing is to move one or the other so there is not a direct reporting link between the two.