<human>: There's a bit of exposition here but I'll try and hurry though it:

Me fiance and myself had dinner at my parents yesterday. When we arrived we walked into a full blown nuclear war between my sister (15) and my mother. Sister had had sex with her boyfriend, mom did not approve.

Right, so later at back at our apartment fiance (26, like myself) remarks "Well, 15 is pretty early". I said "not, really. I lost it at 16 and you at 17 right?"
Which is where the issue arrived: he said "uh, no. I lost it at 23. **to you**. I may have lied, very early in our relationship because I was embarrassed, but have no reason to now."

This blew my mind! We're engaged! I don't think I could be comfortable marrying the first person I had sex with. Sex is awesome, and while I'm loving what I have now, I'm glad I had all of my previous lovers, each of which was different.

He is a wonderful man, who I want to be my husband and father of my children. But, like. I don't know, he deserves more? It's hard to put these feelings into words. I trust him completely and would never expect him to cheat, but what about ten years from now? Isn't naive to think he won't ever get curious? How can I be expected to be the sum total of his life's sexual experiences? The thought of him with another woman is terrible, but I was running on the assumption that he HAD BEEN with other women. 

Is this of less importance then I'm making it? It feels like a big deal and he was acting like it wasn't.

Summarize the given text in a few sentences.
<bot>: Turns out I am the only person he has ever had sex with. This makes me fear for the long term viability of our relationship.