It's actually rather rare for people to defend standpoints they don't personally hold. It's similarly rare for people to bash beliefs they do hold.
It's easy to find a gun rant with sentiments like "You can pry my guns from my cold, dead fingers"; the classic pro-choice catchphrase is "it's my body".
People tend to make the generalization that if they stand up for it, they must be one of them.
Now suppose that Mr. Foo, a heterosexual, doesn't find anything wrong with a person's right to be homosexual. This is not the only opinion people hold. If Mr Foo writes in defense of homosexuals, Mr. Baz, someone from Foo's real life might easily find it.
Let's assume Mr Baz is an active bigot and decides to make Mr Foo's life miserable, because Baz thinks Foo is gay. Baz could even get outright violent. Baz is not a good man.
Mr Foo knows there are people like Mr Baz about and therefore includes the abovecomplained standard disclaimer.
There's other reasons too, but the fear of misplaced intolerance* is the one I can think of.
* Yes, intolerance is bad. But which is worse, being persecuted because you're $minority or being persecuted for being $minority even though you're not?