As you said, he's speaking logically. Of course the homosexual activists don't like this, because they know that if everyone else did the same, they may not be quite so in favor of homosexual marriage.
It won't be long now, in the US, before some polygamists start agitating for their 'right to marry'. On what grounds will a court that has already allowed homosexual marriage deny them to anyone else?
Exactly - and this is why it might be a very good thing if polygamists began boldly requesting this right now (already occurring in Canada).