Gay Marriage legalized in D.C.

[Unreadphilosophy]

Would it be Constitutional for the legislative branch to pass such a law dealing directly with the rights of a particular group?

If you're directing that question toward the issue of gay marriage, then the answer would be no. Under the power of the tenth amendment, marriage is an issue that is only reserved for the states. It is not up to the government to rule on whether or not gay marriage should be legal or illegal. The people of each state need to get together and come to a conclusion on the issue with the people deciding the outcome.