I thought you asked about being a fan of college and the NFL, where they are all adults?Your examples in the first paragraph are different and seem to be a justification for continuing your fandom. It's technically illegal to watch porn and go to strip clubs when you're under the age of 18. Your son would have to be a certain age to legally participate in a daredevil show, and would need a certain level of coordination and competency as a human. There is no age restriction on playing football. Saying you wouldn't allow your son to play football, even if that's what he really wanted to do, and then turning around and showing him the sport first hand and the resources, time, etc you put into the sport is hypocritical. "As long as it's not my son" kind of mentality.
The military part is also different. A more apt comparison is whether or not you would cast a vote to send troops to war, but never allow your son to enlist if that's what he wanted to do (which again doesn't really compare because he can't enlist in the military prior to being 18 years old, without your consent).
There are plenty of things that are fine occupations or avocations but may be dangerous and/or not a good fit for a person. I don't find any hypocrisy at all in saying that one of those things may not be right for someone you know and is in your care while still passionately enjoying that thing. I played football. I have no regrets and would do it again. My son is not me.