I do not really care if people open with "Do you have a boyfriend?" or "Do you have a girlfriend?" Or "Do you have a partner?". I will answer any of those questions by telling them what kind of romantic relationship I do have. Their reaction to my answer is where I would judge their attitudes, but I would not get offended by them starting with any given assumption, no.

There are ways in which heteronormative assumptions can cause real issues and systemic neglect, but there's also something to be said about measuring the amount of emotional energy you expend worrying about any single manifestation of heteronormativity so that your investment is proportional to the actual impact. You'll exhaust yourself and others if you find yourself picking apart every interaction and event, or expecting people to be there already.

The biggest thing you could do to effect things in the way you want is to come out and then continue to be normal. That's how your normalize a thing for others.

There's also the possibility that your grandma noticed your hints and was providing you an opening to correct her without her explicitly asking if you were gay/bi (which might be an uncomfortably direct question for some people to be asked).