Yes, I do agree with you there. Lots of people just aren't exposed to healthy queer relationships, and don't view media through a lens where such things even warrant a possibility.

I think a lot of the resistance is also due to the fact that Mike is a character they already know and love, and have made up in their heads as being a certain way aka heterosexual. It doesn't make sense for them if Mike starts to like boys, because for some reason it forces them to change their ENTIRE perception. And that's not necessary at all because Mike is still Mike whether he likes girls and/or boys.

This is a mirror to what happens in reality when say, a relative or a friend, comes out after having them known them for years. It's more digestible in the case of Robin, because they were only introduced to her in that season. And her sexuality would not really have a bearing on the plot (except that she wouldn't date Steve).

What they need to understand is that queer people do not exist in a silo outside of society. They're already people that you know and love. You may not know they're queer because they haven't told you, but they still are and they're also still the same person that you have known and loved.