My boy...friend?

Ive been dating a guy for almost 4 months. We both know we like each other and have said it in the past, but the other day I asked if I can say he's my boyfriend not necessarily to make it public,but so I can know he's "mine" and he said no Becuase he wanted to ask me to be his girlfriend...officially. I honestly don't get why asking its such a big deal. I don't think asking is important as long as he likes me and I like him and we both want to be together we're together....I hope it's not a bad thing. >.< be brutal!