"You're not serious, right?"

I found out that apparently a decent amount of my boyfriends basically don't believe in us. When they find put that he is 10 years older than me they say "its not serious" or "you're not serious, right?". I just don't understand it. Not a single one of my friends has said anything negative about the age difference. I just don't understand why they say that after they find out the age difference, its not even that big of an age difference. We do think it is weird sometimes ourselves but we are happy and doing very well so what does it matter? I don't understand, I don't know how to react or what do say and do with this information...