I read somewhere that Kerry's father was pushing him to take a public role in the anti-war movement. He made it clear that he thought Kerry was a wimp if he didn't. That is the dynamic no one talks about.
Kerry was a man at that point, well into his 20's. There comes a time in each of our lives that we have to say that I am my own person and responsible for my own actions. Kerry's actions were treasonous and way past just being "active" in the anti-war movement.