Father: Lok'tar! Son!
Son: Lok'tar father.
Father: What's wrong? Were you not able to crush the Alliance beneath your mighty blade?
Son: My blade and technique are fine, father; it is my heart.
Father: Your heart, does it ache? Are you ill? Did they strike at your breast? Do you need to see a shaman?
Son: I am not ill, father, yet my heart aches. I fear there is no balm to ease my pain. You see father, I need to tell you something.
Father: Spit it out, then. What is it?
Son: Father, I'm...Wait, what am I?
'Lesbian' was a reference to the denizens of the isle of Lesbos (now usually translated as 'Lesvos'). Azeroth has no isle of Lesbos, so the word 'lesbian' wouldn't make sense for them to use. In a similar fashion, 'homosexual' is something of a clinical term, one that I wouldn't expect for any Azerothian race to have a word quite like it. Also, 'homosexual' is a pseudo-Latin word, made my Latin scholars to describe people who are attracted to those of the same sex. Latin doesn't appear to exist in Azeroth.
In World of Warcraft, there are several different races who, when interacting with the player, speak English (or whatever the language that copy of the game is localized into, I'll keep saying English for simplicity). We understand as players that this isn't actually their native tongue. We know it isn't, because we often hear them speak in their native tongue for some small things. Aka'Magosh, a blessing on you and yours, is a perfect example of this from Orcish. We know that Common (the language of the humans in WoW) isn't written with English characters, we see that in game. However, we never hear the humans speak anything but English.
The humans may also have an unknown "real language," but we have no reason to believe that the humans speak anything but English (or whatever the localized language is), just with a different character set for writing. Yet, there are still words that we use that wouldn't make sense for them to use. I mentioned 'lesbian' earlier, but there are more. What about loanwords from other languages? It would certainly be weird to hear a character order a croque-monsieur or to slip "Je ne sais quoi" into a conversation because those are very definitely French, even though they are used in English conversations. However, 'gay' (as in happy) is a French-derived word that is very much English. 'Gay' has actually taken on extra meaning since it came to English, meanings other than 'homosexual.' Where's the line between the two? Where would 'baguette' fall? Would it make sense for them to say 'homosexual' even if Latin doesn't exist in WoW?
In the end, the significant majority of words we have come from, are derived from, or are somehow constructed from pieces of other languages. The question is about how thoroughly assimilated into English these words are, and if it would make sense to hear them in World of Warcraft. If humans speak English, then these words wouldn't make sense for them to have. If they don't speak English, then it matters for the reason of player immersion.
Returning to our original question posed by our young orc friend. I think the right word would be 'gay'. I've chosen 'gay' because it's meaning of 'homosexual' came from society's collective agreement and use of it for that meaning. But that opens a whole can of worms about how homosexuality is viewed and treated in orc culture. Do orcs view homosexuality as a weakness? Would that be allowed in Garrosh's Horde? Or would homosexual sex be okay but someone still wouldn't have a lifelong homosexual relationship in the same way that one would have a lifelong heterosexual relationship? What would each race's views on homosexuality be? Would that be communicated in game? Would it make the most sense for orcs to be homophobic but to avoid the issue in-game to keep ammunition out of the hands of homophobic players?
I think the issue of how the orc and other cultures perceives homosexuality is one that's best not touched upon in World of Warcraft. Homosexuality is something that many players use as an insult and as a means of discrimination, and the realistic answers to how many of the in-game cultures would perceive homosexuals probably wouldn't help the matter. I still think there should be homosexual characters and that they should be received well by their friends and family, but that the larger, cultural issue should be left alone.