Canon is changed all the time, but more often than not, there are things about the character (whether they be characteristics or people or locations) that have consistently stuck with that character for the majority of their existence. Those things are typically considered sacred (for lack of a better word) and shouldn't be changed without very good reason.
Is Spider-Man being heterosexual one of those things? Hmm...I suppose. I mean...his love interests have all been women. The love of his life is a woman. The character has been understood to be heterosexual for the majority of his existence. So...why change that? For what reason? To make a statement? Why can't they simply create a new character? Why does an already existing character have to be altered just to prove a point?
I'm not against change. I think change is good. Sometimes, it's necessary. But, honestly, it depends on the character and the change you're making to that character. It also, I think, depends on what you, as in individual, are comfortable with, and how much you care about the character in question. I mean, despite everything I just said, I wouldn't blink if the next Spider-Man was black. On the other hand, I would be upset if the next Superman wasn't an alien from Krypton.
I don't know...when it comes to things like this, I don't think there's one right or wrong answer. However, it should be said that intent should play a huge factor in the response these decisions receive.