15 year age difference.

My fiancé and I are 15 years apart. I am 21 and he is 35. Turning 36 this year. He has a 15 year old daughter, I know it's weird but we get along surprisingly. Every where we go they always think I am one of his daughters if his daughter is with us. When we are alone and his daughter goes over to her moms we never get that. But today we got it a lot and I could tell it made my fiancé think. We got home and asked him if it had made him feel bad and he said he just gets a little embarrassed because he thinks they think he is a creep. I know a lot of people say what could we possibly have in common ? Well, there might not be A LOT but there's definitely something we see the same. We obviously don't think the same as we are both in different levels. We also have a 8 month old baby girl. We are engaged and we get along very well. But it really hurt me when he said he's a little embarrassed about it. What can I do?