NAH. I used to ski when I was younger, but there are a lot of risks. If you've never gone skiing, and don't know what you're doing, you can get into some trouble. Unfortunately you can't discount the fact that he is a minor, and while it sucks that he doesn't want to bring your boyfriend, at the end of the day it's a huge risk to him. If something happens, which is 50/50, because again, skiing isn't exactly the safest when you are inexperienced, his parents could very well destroy your father.
I can understand where you would be frustrated. Most states, including Utah, have a Romeo and Juliet law. You being 18 wouldn't be an issue in Utah because his parents are clearly fine with this, and you are within the age range allowed by their laws. People citing your ages as an issue are right to be concerned, but it would be a non issue in Utah.
The safety of your minor boyfriend, on the other hand, is a big deal. Going on trips to the beach is one thing. I remember being a teenage girl of 14 and thinking that I would just slow down after graduating from the Bunny Hill to the next biggest hill. I didn't, kept going, flipped over a post and slammed into the front of our school bus so hard nobody knows how I made it out without broken bones. Teenagers are stupid and make stupid choices because they think they're invincible. Without the guarantee that he wouldn't do something stupid - and with teens there is very rarely that promise - and get him into a ton of legal and financial trouble.
I was the quiet, shy, straight A, never did anything daring or dangerous kid which is exactly why my parents decided to let me join the Ski Club, and after that incident I was never allowed to go back.
You're allowed to be upset, because yeah, it sucks, but your dad is allowed to be cautious when dealing with someone that is considered a minor no matter where you go. He's right, and is doing the right thing, but you can, and are absolutely allowed to, be upset about it because you're right. Other people are bringing their SOs. Ask yourself if you would want to take a minor on a trip where he could break his neck, as a parent and seasoned adult. ( Like think late 30s and into your 40s. )