While it isn't why she spoke out against it and she seems to be less-than-progressive about a lot of things (which isn't terribly surprising for a previously largely unengaged 65 year old), it's probably worth pointing out that a lot of queer people consider gay marriage to be itself less-than-progressive.

The more radical queer wing (arguably more aligned with the goals of early queer political organizing) pretty frequently comes out against gay marriage, or at the very least against how it was made a priority over other fights like employment, suicide, mental health, youth homelessness, access to medical care, etc., largely championed by and for affluent white gay men, and by corporations intent on using it for PR to sell themselves as inclusive and progressive. And the latter part is uncontroversial - funding for gay marriage and the initial groundswell of support came from a fairly small number of relatively conservative sources and involved a lot of astroturfing. You can look at huge organizations like the HRC, which have a really checkered track record with anything other than mainstream gay men and lesbian women - at one point even explicitly saying that bisexual and transgender issues took a back seat in their efforts compared to gay marriage.

The larger idea is that gay marriage represents a negative kind of assimilation that sees us joining repressive power structures rather than our more traditional stance of questioning them or rejecting them - that we are merely adding "normal all-American gays" to the list of acceptable kinds of people rather than doing away with the notion that people should care about the form of one another's consensual relationships or sex lives that they're not party to, and that their cares should be afforded legal status. The (I think pretty reasonable) fear is that nice gay couples with little dogs and picket fences joining the fold doesn't actually help people who aren't themselves already very close to the acceptable mainstream - if anything it's made the "freaks" even less acceptable, now that the nice gay couples with little dogs and picket fences have joined the mainstream that still disapproves of the "freaks", and everyone else also feels exasperated, like they've done their part in promoting tolerance and really, how much more can be expected of them.

There's also a really significant degree to which many people feel like the gay rights fight is basically won and over. Making it all about marriage has definitely hurt ongoing support for other queer issues now that gay marriage is legal. I've seen countless straight friends crusade for gay marriage, but now, even as there are so many other issues, they're basically silent. The idea that marriage would serve as a sort of "stepping stone" to more important issues doesn't seem to be panning out.

If you're interested in learning more about this perspective, this is a pretty nice article I've sent people a few times: http://www.organizingupgrade.com/index.php/modules-menu/beyond-capitalism/item/1002-marriage-will-never-set-us-free