The on going talk of Marriage...

[TwilightDisciple]

And then there's goblin lands. They're like the Japan of Azeroth, anything goes there.

Except they're a little more public and realistic about it, unlike Japan.
Silly Japan.

[Theik]

Everything I've read on trolls says that they will take multiple partners (specifically, the males will take more wives), but only if they can support them. So, if you're a well-to-do male troll, you may have more than one mate. Trolls also may give away sisters or daughters in order to cement alliances. All of this is non-Darkspear or pre-Horde Darkspear lore, though.

Many Darkspear traditionalists don't agree with that though. In the Horde Player Guide, Brann travels with a Darkspear traditionalist for a while, and the guy admits that there are many Darkspear who still practise voodoo in secret, and that many still have multiple wives.

Remember that Thrall never said they had to practise monogamy, they just have to start treating their women with more respect.