You sound perfectly reasonable, I think.
As far as any children you have in the future, of course you have a say in what they call your parents/steps. It will be up to you to decide how much time your kids spend with each, and what kind of contact they have. You may decide later you are happy enough for your kids to have four grandparents on your side, but you certainly don't have to toe any kind of line on that score. Obviously you can't control how your future kids will feel about it all - but you do have some say in how it develops.
My husband's dad had affairs, and has had three wives, including my DH's mother. She is deceased, but the other two wives are alive, and wife #2 is the mother of my husband's half-siblings, so she is still a part of the wider family. Technically, then, my little girl has three living 'grandmother' figures - my mum, DH's first stepmum, and DH's second stepmum. But only my mum is called 'Grandma' - DH and his older kids always call first stepmum by her first name, and that's probably what my daughter will call her, too, but everyone loves and feels close to this woman, anyway - the label does npot seem to be the most important thing. My DH says that he never thought of her as a MOTHER, but he loves her for who she is, and his older kids do describe her as their grandmother, even if they don't call her "Grandma". Second stepmum is also called by her first name, but never talked about as a grandmother figure - and she hasn't ever even acknowledged my daughter really, so I don't think she is interested in having that role.
My confusing in-laws aside - I guess my point is that families are really sort of made up of whoever you WANT included, once you are an adult. You may not have much choice now about the shape yours has taken, and I trust you are doing your best to accept things as they've been, but I would try not to let your mom and stepdad's assumptions about the future bother you too much.
You may well end up feeling like "the more people my child has to love him or her, the better!" Some people do. But you may decide that you'd rather have some limits, and I think that is ok, too. I don't know what your relationship with your stepdad is like, of course. You are moving on into an adult world, though, and the relationship is bound to change, hopefully for the better. You will find what works for YOU, in the end.