You know that guilt by association really is pointless? Why dont we hold all the German citizens during WWII as guilty of war crimes?
If you quote the whole paragraph instead of the first line, you'll see that her point isn't about guilt by association:
Because on something this important and big, I consider you guilty by association. You are paying tithing dollars, fast dollars, etc. to a Church who put money, time and effort into trying (and succeeding) to not allow gay marriages. You are part of a church that had their members give their everything (because of Church teachings) to fund "Yes on Prop 8".
It's not about being blamed for someone else's actions; it's a matter of freely giving material support to (IMO) an evil act perpetrated by your church against other people.
IMO, anyone who donated money to any church or other organization knowing that it would help to fund the anti-same-sex marriage campaign does hold some degree of responsibility for what their organization has done.
Maybe you can justify this by other good work that the LDS Church does with its donations, or by the personal cost you would've borne in your religious community if you stopped tithing, but the harm that this money did to other people certainly does factor into the morality of your decision to give it.
Now, I'm not saying that Mormons are guilty in a way that other anti-same-sex marriage groups aren't. I don't think that the LDS Church is the only bad guy in this; the guilt is spread across many religious denominations. Still, someone who funds hate and discrimination should not be surprised when they're called to task for it... and the members of the LDS Church, through their tithes, (along with the Catholics, many Evangelical Protestants and a boatload of smaller groups that aren't immediately coming to mind)
have funded hate and discrimination.