It's correct that sexuality isn't mentioned in Prop 8. While the de facto discrimination against gays is a sexy red herring, the de jure discrimination is against gender and I think that's where the legal argument really is.
I look at it real simple-like:
There are privileges provided by marriage. First things that come to mind: tax benefits, 401K benefits, inheritance, medical decisions. I believe those privileges shouldn't be disallowed to a consensual adult relationship--marriage, civil union, I don't give a **** what it's called--simply because of the gender of both parties. To me, that reeks of gender discrimination, and as a violation of the Fourteenth.
If SCOTUS strikes Prop 8 down, then DOMA will go, too. We're watching a landmark case unfold.