You are not entitled to a platform. The student body choosing who they want to listen to is, in and of itself, a form of speech. Why is this such a difficult concept for people to understand?
The difficulty with rights is where they start and where they end.
The saying goes that my right to swing my arms ends where your face begins.
Equally the student unions right to decide what they personally listen too ends when they prevent others that want to hear the speaker from listening.
At that point they are infringing on another's right to free speech.
In the case of the student unions it is an inherently political organisation using their power to restrict the rights of what I can only assume is a minority of the student body.
If the student body doesn't approve of the student union, do something about it. The student body refusing do something about it is implicit speech that they approve. If most of the student body isn't changing the student union, that tells me that perhaps your identification of who is in the minority here might not be accurate, or is at least incomplete.
> your identification of who is in the minority here might not be accurate
Are you seriously suggesting a students speech should be curtailed because they are in a minority! (Or a majority for that matter). You are either trolling, or you are part of the problem.