I hate to start this up again, but...

[AmirioTheMuzzy]

This brother on Reddit needs help...

Let's focus on the etiquette [adab] of Mutah as described by Rasulullah (S) and his Ahlul Bayt (A).

Let's NOT focus on the rulings [ahkam] (we have thousands of threads on this already, we all know the rules by know, and it's not hard to figure out the rules anyway, all you have to do is check your Marja's risalah)
 

 

[realizm]

Things were like this in Hijaz during the time of Rasulullah (sawas). 

Man would divorce their women without consent, many had concubines from amongst their slaves and some would engage in temporary relationships with women.

The book of Allah (سُبْحَانَهُ وَ تَعَالَى) legislated thoses practices with rules. Amongst them, mut`a.

What would be the etiquette for marrying your slave girl nowadays ?

It is just the same logic and the question needs to be thought upon with the context of the era we live in.