It seems clear that any intelligent, informed, honest and public spirited person today will be racist, sexist, homophobic, antisemitic and xenophobic. That is to say, he will accept that ethnic, sexual, religious and similar distinctions and connections matter and have always mattered to people, and there are good reasons why that should be so:
- Sex has been around for a billion years. It seems to affect people, and they seem to think it's important. If that's so, why shouldn't it be thought to mean something? And why shouldn't people have customary ways of dealing with sexual connections, distinctions and meanings? Such claim, though, are sexist and homophobic.
- As a practical matter, culture is generally connected both to religion and to ancestry and thus race. Culture however is a network of common attitudes, beliefs, habits, loyalties and the like that grows up among people who live and deal with each other for a long time. As such, it facilitates cooperation. To say "diversity is a challenge" is to say "diversity causes problems." It makes it harder for people to cooperate.
Why not avoid problems? Why not at least recognize that they are problems? If culture has a function, and cultural differences actually exist and sometimes cause problems, then it must sometimes make sense to prefer to associate with people of one group defined by ethnic and religious background rather than another. But that's racist, antisemitic and xenophobic.