I haven't read the rest of this thread so I'm not sure what advice you've been given so far, but here's my take. I'm 20 years old, going on my first vacation alone with my BF of 4 years this april. When we were both 18 I was in the same situation as you, my family was going to WDW and my mom mentioned that I could invite my BF!!
We invited him and he was going to come, but his lacrosse schedule got in the way so he never came. But I think the reason my mom wanted him to come wasn't to please me or try to keep me happy or anything, but because he'd never been before and she loves taking people in their first trip!!
When my BF couldn't come I was upset but I wasn't miserable the whole trip, in fact I wasn't miserable at all, I brought my laptop so I could send him e-mails every night.
I don't understand what the issue would be with bringing him, I don't know what would be inappropriate about it to be quite honest
If sex if the issue you're worried about, take it from someone who was just 18 two years ago, sex would not be the first thing on my mind while I'm at Disney with my family (especially if we're all in one room!!)