I can't help it

Or doing a project in school.

My mother says it because her and my father are not yet married and I guess it sounds better than saying my boyfriend when you've been together for 15 years and have a kid (also fiancée works in that scenario as well)

I do too. I'm not homophobic its just usually what homosexual people say.

I definitely question that someone might be homosexual if they say "my partner" but not everyone does. I've been being purposefully ambiguous about my relationships at work, and one woman I work with has definitely not caught the fact that I am persistently gender neutral in response to her referring to my ex (who is female) with male pronouns.
That being said, I told someone else "my roommate " and her immediate response was "like your 'roommate' or actually just your roommate". So definitely some people are just more inclined to pick up on that kind of thing.

And when people make assumptions like that, I assume that they are homophobic... and probably racist too.