Well unlike the 1964 Civil Rights Act where making gender a protected category severely backfired on the conservatives, I really doubt adding support for gay marriage would backfire on them as well, if anything it would just kill CIR.

Bottom line: If you want immigration reform at all - no gay issues in it, just legalize under it like anyone else and if Supreme Court strikes down DOMA then go through a marriage petition, since you can be sure USCIS will allow that.