Is it normal to think I will feel closer to my own child than to my step-child?

I asked a question earlier "should I buy my boyfriend a father's day present?" He has a child (12y.o son) with his ex-wife. I didn't wnat to buy one until he was the father of my child, I wanted our first father's day together to be a special "first moment together" for me and him to share. A few people who answered my question (thank you!) remarked that if I felt that my children would be "more special" to me, I am not ready to get married . I do not expect for my bf to feel that way, but, I can't help but think that I will feel closer to the babies I give birth to than the teenaged boy I see once a week. Some people remarked that I should "grow up" and not feel this way- that if we get married I would automatically be his son's mother. I disagree, his ex and I get along very well and I feel that my place is to be a supporter and a mentor and a friend, not his mother-he has one of those. Is this wrong- am I in fact, "too immature to get married" if I think I'll feel this way?