It doesn't matter whether you support or oppose gay marriage, this is an issue of law, current law.
The state Attorney General is substituting her own preferences to Pennsylvania law, which she is sworn to uphold. Ms. Kane doesn't get to decide constitutionality, the courts do that.
This is materially no different than George Wallace blocking the entrance to a school because he didn't agree with the court knocking down segregation. It is different only in that we don't like where he was, but (most of us) do like Kane's position. But that it not the issue. The issue is obeying (and in Kane's case) defending the law, even if not palatable.
(Frankly, couldn't she just have assigned a low-ranking, inexperienced attorney, who would botch the job? She could have. I think she is show-boating here.)
If doing her job creates an ethical conflict for her, she should resign.
Read today's Daily News story on Kane's decision here.