I can see why all of that would lead to you feeling anxious. Would it maybe be helpful then to plan what you're going to say to him if he approaches you about this topic again? Too, in more general interactions, is it reasonable for you to take a polite but cool approach? In other words, you say hello back if he says hello and things like that, but you aren't inviting his company.
As far as those fears about other people assuming things about your sexuality because you fall silent or leave certain conversations, it could be that some people are making assumptions. But people are often way more self-focused than we suspect so it's equally, if not more, likely that they're not picking up on the pattern.
I do want to touch on your feelings around being fluid. If that's the label that feels right to you, or feels like it accurately describes your experiences, then it's yours to use. But if bisexual feels right but you're worried about it being disrespectful to use, it may help to know a couple of things. One is that the majority of bi people don't experience attraction to men and women (or any other gender) in exactly the same way. In fact, we have a column where we address a user who's worried they don't "count" as bi because they don't feel exactly the same about men and women: https://www.scarleteen.com/article/advi ... _and_women
Another thing to know is that, if bisexual is what feels like a match to you, you're not disrespecting other bi people by using it. Bi experiences are varied, so most bi folks know that just because their experience of it doesn't mean another person's isn't valid and vice versa. Does that make sense?
With your mom's comments, is there any accuracy to them? In other words, has your dad chilled out around this at all if you don't engage with him. Or has he continued to be very pushy and demanding?