Earlier quoted context omitted.
To most Americans, $1k is not a "minor" amount of money. Furthermore, the exact amount of money that he donated is fairly irrelevant anyway. The donation, no matter it's magnitude, serves to illustrate what he believes. If I made a $1 donation to the local KKK group, would you make excuses for me because $1 is a trivial amount of money? Of course you would not.
Comparing donating to an active hate group to a campaign doesn't make sense. The amount is irrelevant, I'll give you that but in my opinion something someone did six years ago isn't going to determine my opinion of something now. In six years time Brendan could have gone through a lot of experiences that changed his opinion on marriage and based on his statement I would have given him the benefit of the doubt.
If Brendan had publicly released a sincere apology I would consider forgiving him. All he did was issue a bunch of "It doesn't matter, you guys are wrong for caring" statements.