Let's look at the simple question first.
This use of the cause is about a principle or idea that is very important. We often use it when we are talking about people who are being treated badly or unfairly. The cause is the idea that they they need to be treated fairly. This is the definition from the Cambridge Advanced Learner's Dictionary
C1 [C] a socially valuable principle that is strongly supported by some people:
- They are fighting for a cause - the liberation of their people.
The sentence means that Freddie Mercury was not politically active in this way. He did not officially fight for gay people's rights.
'I am what I am. So what?'. And that in itself for some was a statement.
This sentence means:
- That in itself was a statement for some people.
The word statement here means "political statement". The word that refers to the idea 'I am what I am. So what?'. The reason that some people though that this was a political statement, is that it means:
- I am what I am. I do not need to justify it. I do not have a problem with it. I do not need to explain it.
Because Freddie Mercury was bisexual, many people might think that he needed to explain or apologise for being bisexual. He didn't do this.
The question mark [ ? ] at the end of the sentence, So what?, is like a challenge to the listener. It is saying: "Do you have a problem with that?".
Hope this is helpful!
Cambridge Dictionaries Online: "Cause", Noun. 14th December 2014