Putting it right out there...
This is
behavior on your behalf. It just is, it is inescapable.
Let me break this down in a way you might possibly be able to comprehend.
As the DM, you control every aspect of the world. So don't go blaming what was written in this or that book or whatever. You, yourself, and only you in the entire universe, get to control what is even considered a "monster" in the world at your particular table. You decide how the NPCs react to others inhabiting their world based on physical traits.
Now, get this through your head. When you DM, you are there to make a good story WITH the players. They are not your opponents. They are not your enemies. They are not your rivals. They are your GUESTS. You are there to entertain them by putting forth a setting and setting up a challenge and curating and refereeing their experience of telling the story they collectively want to tell within it, with as few boundaries as possible. Your job at the table is to say "yes, and..."
Your players are not there to hear your story from you. You want to micromanage every aspect of the PCs actions and choices and possible experiences down to the single one you approve of? STOP PLAYING D&D!! You clearly just want to go write a book-- so go write a book and then you can hand it out to as many people as you like. Far more than the few players sitting at the table. The players did not come to your game to be strung out and tortured and abused by you for having had the gull to have dared to have played the game in any way that you don't personally approve of. You want to do that, go join a BDSM group-- but be warned that even they have safe words. When you find yourself at the table shouting "NO!! SCREW YOU!! AND I'M GOING TO TEAR YOU TO PIECES FOR BEING DIFFERENT!!" you've lost the script, you've **-*ed your whole game and you need to turn around and rethink your entire approach.
You want to know why a lot of players want to play something unusual. Because everyone here has no choice but to be a human every single day of their lives. Everyone they meet is a human. Every person they have ever seen or talked to or had any experience with has been a human. And now here before you sits a world with beings of all shapes and sizes, dozens upon dozens of new sorts of people that have sprung forth from people's imaginations over nearly 50 years of this game existing.... And then you are going to say that the only valid options are human, child-sized human, fat dumpy neckbeard human or gaunt super-model human? What a load of crap!! Maybe that's fine for your shrunken, miniscule imagination who decided that "Diety beyond Dieties, Master and Saviors, Lords Tolkien and Gygax only penned these choices alone to be heroes within the holy sacred volumes from which we cannot ever deviate" and thus decide that all the thinkin' that's to be done was done 60 years ago....
But you know what? Everyone playing the game right now has come up on Elder Scrolls, WarCraft and EverQuest and dozens of other games in which being those other shapes and sizes of people were perfectly valid options and one could be one and be accepted and be a hero, more and more so as these games and those that took after them went on-- few questions asked or limitations placed, the denizens of the world would not assault you without reason or warning for no other reason than existing. And quite frankly, it takes considerably more work to allow one to play one of those other choices in a fully rendered graphical game experience than to just jot down a different word by that "race" field on the character sheet form and having people react differently when it mattered. So why exactly should D&D be so god damn regressive and self-destructive as to artificially limit player options. And the really funny thing is, it strikes me that it is quite a lot less disruptive to the concept of the world to allow players to play the people who are already well-established as existing within the world and already are going to be regularly encountered, even if as primarily or exclusively antagonists, than all the various ass-pull "new hotness" creations that WotC has come up with since 3.5-- Warforged, Dragonborn, Tieflings, Bladelings, Shifters, etc... where basically the players are basically going to be one-of-a-kind in the entire world and well.. often enough, considerably more alien and monstrous than the "monsters" that already exist there.
Honestly, it is unlikely that any of the extra choices are ever going to be competitively popular with the simple mainstream Tolkien inventions (yet another reason why it is so unlikely that one's favorite is going to ever be playable in any format outside of pen and paper table top where they can only exist in the collective imagination of the group), but as they are easily implementable, then they should be implemented-- with the only reasonable limitation perhaps being that they should not expect access to powers and abilities so far reaching that the challenges set forth become trivial and the other players feel that they are no longer able to meaningfully contribute to the success of the goals laid out in the game and have merely become spectators along for the ride.
Oh, and simply because a creature is "evil" does not mean it cannot and will not be an ally to the supposedly "good" (though more often than not, neutral) mixed race but human dominated civilization at the centered of the game world. Nor does a race being "good" or "neutral" not mean that they won't be enemies.
A human capital could well have a working relationship with the savage orcs who live to the north and regularly provide them with food in exchange for furs and the added bonus that the orcs are going to fight off anything nastier than them that tries to make its way south. Or they could be long-lasting allies with a Hobgoblin fiefdom to the east and while they find their ways savage and don't agree with the way they keep slaves, both civilizations benefit from one another and aren't prepared to go to war over such cultural differences. And just as possibly, the main civilization could be heavily deforesting the forest or strip mining the mountains in order to get the resources they need to meet the needs of their people and expand their lands and wealth... but, as a result, have come into conflict with the elves (and dryads and treants) and dwarfs.
You really just need to use the slightest bit of common sense and imagination to make any situation work.
But, fundamentally, if you find that your players want to play something outside of the ultra conservative, limited scope that's been done to death and worn beyond thin over the last 60 years... then the last thing you should do is punish them for it. If they expectation is that they should be accepted by society to whatever extent, it is your job as the DM to say "yes" to that and think up the reason why they would be. Maybe the town is quite used to people having goblin or lizardfolk slaves or servants and so ultimately just doesn't object so long as they are with the other party members. Or, just simply, that the city has all sorts of shapes and sizes of people coming through here constantly and go ahead and fit those same races into the town as NPCs in the background. And while 90% of the goblins and lizardfolk you might meet out there in the wild will effectively be bandits and cannibals, over the literally tens of thousands of years that this world has existed-- inevitably however many tribes got conquered and absorbed into the general civilization.
If you are too stupid, too mean, or just generally too crap to be able to do this... don't be a DM. Just quit. Explain that you are so bigotted and small-minded that you cannot help but force your personal preferences on everyone else and are driven to assault anyone who has preferences or ideas different from your own. You have however many other people at the table who can do the job and absolutely certainly do the job better.
