EDIT: I've gotten a lot of great feedback, thanks for all the suggestions guys!

TL;DR: Starting a new Campaign (5thEd) with new players, and a few of them want to make 'out of the box' characters. I keep getting asked if Volo's/EE supplement races or Homebrew Class are allowed. I want to say "No." but I don't want to be a prick about it. Halp?

So I might be an outlier here, but I've always been a bit of a purist with DnD. I think a lot of my bias just comes from how I was introduced to DnD, and I've noticed I tend to emulate my original DM who I played with weekly for 6 years. Generally my thoughts are "But why?" Personally I find Dragonborn & Tieflings to be a little silly, and don't really see a way that they fit conveniently in the relatively low magic games that I run.

Right now my main issue is, I'm starting a new campaign with new people who I don't know. A few of them are DnD vets, and a few of them are fresh meat. A couple of the newer players seem like they really really want to play an 'Alt-race' or a funky class, for no particular reason other than it fitting their fancy, which I understand from the Shiny New Toy aspect. For me however, I find this almost annoying, and I'm not sure what to say. I haven't straight up just told anyone "No." yet, but I have asked people to justify why this race would fit into the game setting. A couple of them seem to still be trying, but we haven't even discussed setting or the game world yet, (I have multiple games/modules I'd be interested in running, so I want to hear their opinion.)

We're doing a Session Zero to hammer out Rules & a Social Contract for the game tomorrow, and this is something I want to address. I'm just not sure how to bring it up without hurting anyone's feelings, or being a huge dick.

Right now I'm just looking for advice, helpful tips, reassurance, that sort of thing. I guess the last thing to tack on here is, I feel like I'm really trying to be reasonable and open minded, but I also don't want to have to re-work a world to try and perfectly fit in 7 different player's races/funky classes and back stories. So I'm having a hard time with that.

Thanks for reading guys, cheers!