Novel slavery...

Slavery is not bad thing if it's jp novel
Here some example

1. Slave can eat together at the same table with master, not in the floor
2. Master will treat slave nicely (polite talk, ask slave opinion, order slave to sleep in warm bed while master sleep at the cold floor)
3. Master will save slave in dangerous situation even there is death risk 

How else would the MC get a devoted beautiful elf slave-wife with no effort? Just buy her and treat her better than others have, and she's all ready to ride his dick. If more people were annoyed at the setting and didn't read those novels, you wouldn't see this setting anywhere. The fact that you see it everywhere should tell you enough. 

Your last point is not clear. Is the bolded text a typo? 

Everybody gangsta
Until elfinsists start asking for elf rights 

Okay most people said that human race breed like crazy and the other races are being outnumber, and my question is this, are the other race just sit on their fucking ass and did nothing?
The elves is said to live longer, did they become stupider as they aged... They seriously cannot think of any way to solve the problem of being outnumber?
New magic, new weapon, form a coalition with the other race or so on. They seriously cannot think any of this? If they really can't then the only thing that i can said is that their life is wasted.
And about pride, what, being a slave is better than be dishonored? Seriously?? 

The issue is that elves aren't stupid, usually the opposite in fact. And archery and magic make for just about the best ambush tactics in any setting. How long would the elves watch on? Not even a decade, despite their long lifes and apathy. Villages would dissappear deep into magic forests where entire fortress cities would be build. Legions would be formed and every elf would learn martial arts and magic to defend himself or herself. The elves are usually one with the land, and in a medival tech level setting nothing is more lethal than that if it is used for war. And it takes little magic to lay waste to medival economy. Poison wells, burn and salt the fields etc. 

Fertility is the excuse normally used
Since humans outnumber elves a gazillion to 1 

The most simple reason actually. Have you ever heard of the saying 'Deus VULT'. In this case its the author.