It isn't a protest for their cause. There isn't a cause as it is recognised as part of society in this country that you do not discriminate based on, among other things, sexuality. It is, literally, the law. The armband is supposed to celebrate that and indicate that this where we are as a society. No different to wearing an anti racism armband.
The second part of your point is why I would understand if he spoke to his club and they withdrew him from the game. Him playing in it and not wearing the armband is in effect a protest against not being homophonic, amd what you are doing by defending it or saying its not a big deal, is asking people to tolerate that.
No one can control another person's beliefs or dictate their reasoning, but if someone in this country, for whatever reason, doesn't want to support equal rights for others, it is up to them to tolerate that the place they live does, not the other way round.