Question on some negativity here

I’m not naming them due to conflict but , someone replied to one of my questions regarding how my bf had 3 condoms but down to two ( reason is he gave one to his brother who has sex a lot) but then I said we were each others first ( he couldn’t date until 14 ) has soon has he turned 14 he’s been with me and my first same goes for him but then they said “ hate to tell you this but your not his first “ … why assume ? We’ve been dating for 5 years. And … he’s literally mentally not all there to even think about cheating or ect .. but we are each others first .. so I don’t know why they assume when they arr an adult woman who knows nothing of our lives we are still young 18 and 20 .. been together since we were 13 and 14… not going anywhere