Just ask them “why not?” and see how they fail to find a good explanation 
It’s sad that most people still think that characters that aren’t white and straight need to have a reason for being so, while you never have to explain why a character is straight and/or white 
The only time when diversity is forced in my opinion is when you can clearly see that it doesn’t make any sense for a character to be there and that the author only added that character so they won’t get criticised for not having enough diversity