Discriminating by age could be considered reasonable (don't want a large group of college students) and I see valid reasons to discriminate by gender (Women may not want to book male guests if they're sharing the same space and vice-versa)...
You'd still run into the problem of real-life racists (a problem for minorities everywhere, all the time)... but such a policy would likely eliminate many racists from the site through the frustration of not being able to discriminate.