I think it's weird. I've heard two reasons for it:

• Concerns over being dumped for someone of the opposite sex: this one I understand but still find pretty sad. I think it's better to evaluate people's character on an individual basis rather than blanket-ban an entire group.

• A desire to be with someone with whom you share a sexual orientation: this one I don't get. I understand that a lot of people (maybe most people) want to be with someone of the same race*/sexuality/etc., I just don't understand why. Okay, you likely will have shared some of the same experiences with them; why is that better than being with someone who has had different experiences?

However, I also think that at the end of the day it's none of my business and that no one should be shamed or pressured into "deconstructing" their dating preferences just because I have different ones.

*Edit: actually with race it could be a matter of physical attraction, which is different.