For example do you think spray painting "Gaz Love Claire" on a set of shutters should be treated the same way as "(Slur) Go Home"?
We're allowed to not only think, but say or write, all kinds of things. Spay painting them on people's homes is not allowed, Nor is harassing people.
But the idea that something like this will only be applied in these very unsavoury cases is pretty naive I think. What about when someone writes a latter critical of SSM in the local paper, or protests about immigration in fromt of a government building? Or says in a university class that they think women should be encouraged to stay home with infants rather than their dads? Or says at a Pride event that surrogacy is wrong and gay men will just have to deal with it?
Or has some other unpopular opinion that they are putting out in the public discourse?
There is no legal requirement that ideas be put forth in a smooth and kind way, even if we think they should be, and slurs aren't actually illegal to use although they may be against the rules in certain settings.
When you make the spray painting illegal also on account of the content, you are conceptually criminalizing the expression of certain content as well as the other element - in this case vandalism.