What's the big deal with age gaps?

First let me say, I'm 17 and I live in Texas so by law I have the right to choose who I'd like to be with. I've been dating a guy since my sophomore year even though I've known him since we were in elementary. He just turned 21 And we're engaged and expecting our first baby (not planned). People always tell me he's wrong for being with me bc I'm only 17. But I know for a fact, no one will think twice when we're 20 and 24. Why is it so taboo now? I definitely understand the stage of life difference and not to be cliche but I raised my two younger brothers and moved out at 15 to help save my mom money. So I'm mature enough to know what I'm doing, how the world works and I'm fully aware that this relationship could end today tomorrow or after we've been married for 10 years. Other than that, what other things make age gaps so bad in society?