Is it normal?

Yesterday me and my boyfriend were talking about what we expect from long term relationships etc. and I told My boyfriend of 4 yrs that I just want someone who is certain that they want me and only me, and he said that “well that’s not possible, you can never have that, 100% feeling of certainty”. And I sort of got very upset because I am certain that I want just him and no other man. Is it normal to have long & healthy relationship yet not be fully certain? Am I overreacting?