C’mon! Vince probably had no ideal that the coordinators of the Rose Bowl were going to do this when they asked him to be the Grand Marshall. It’s not like he is in charged of picking every float, band and guests for the parade. It’s just a ceremonial position
Probably some people think Vince is endorsing homosexual marriage by his presence at the parade.
Alternately, some people probably think Vince should walk out in protest. I don’t know his opinion of homosexual marriage. Clearly the parade is making a political statement, but doesn’t necessarily mean he shares that opinion.
If Vince walked out in protest, then he would cause a firestorm of protest against himself. He would get the Duck Dynasty/Paula Deen treatment by the arbiters of political correctness.