I agree. One can love and support the family member without approving of all of their actions. If he were best man, that would be another matter. If he'd done something like Senator Portman did, which is change his political actions because of the relative, that too would be an issue.
Plenty to scrutinize Walker about without delving into his family.
It used to be I would have drawn the line at attending the celebration but would have included the family member in gatherings. Now I would draw the line and include the family member ONLY without the other person. Even then I would only include them rarely. I could invite the person to lunch by myself but I would no longer be party to a gathering where other people, especially kids, might be influenced to lower their guard against this evil. It is a disease the infects others. When society rejects it you can more easily show mercy. But when the pressure is to accept the sin, mercy means harm to others. In that case you have to choose protecting the weak. The wayward family member did it to themselves and could fix the problem by simply turning from their wicked ways.
And no it is not like every other sin. This is a sin the government is forcing people to accept and celebrate. Other sins can be ignored and the family member usually already knows/admits it is wrong, or at least respectfully declines to flaunt it.