I, F(21), am about to have 3 angry roommates. I don't want to go on the college senior class trip. Help?

I, F(21), am a senior in college, and my school just announced their senior class trip. The thing is, there is about 700 people in my class, but they are only allowing 60 people to go (one bus full, 13 hotel rooms). They announced the trip Friday and they are starting to collect money on Monday. Everyone who wants to go on the trip will be rushing in on Monday to pay their dues. I am not a fan of the location they are going to (it is to a casino/club city), and it's during the off season (in January) so nothing else but the casinos and clubs will be opened.

I hate clubs and casinos, and I know I will not enjoy my time there. I also know that when you sign up for a hotel room, you need all four people there to sign up. (so 3 people can't sign up hoping a fourth will fill in later). I know when I tell my roommates that I don't want to go, they will be so mad at me. They probably won't have enough time to try to find a fourth person. Clubs are their thing and I know that they will want to spend their whole time doing that, but it is not mine. Plus my boyfriend of 3.5 years will not be able to go on the trip as well (he can't find 3 other guys to stay with, none of his friends want to go), and I would rather just enjoy the trip with him.

My boyfriend and I love things outdoors and museums. (I will also have to say that my boyfriend is not a fan of the location of the senior trip) So, today we created our own little trip which will be happening at the same time, and we can visit his family on the other side of the state. I know I will be much happier going on our trip rather than the senior trip, and it will cost the same amount of money.

I'm really afraid to tell my roommates this news. Again, the college announced this trip on Friday, two days ago. I never told my roommates a defiant yes or no. I am going to be telling them a NO today though. What is the best way to approach this?

Just a note, my roommates are sometimes immature when handling stuff like this... there is a chance they won't like me the same way again.

Update: Two of my roommates just came into the house. I went down and told them that I'm sorry I'm not going on the trip. That it was a last minute thing that our college put on us. Basically they said "That's fine", "Whatever". It seems too good to be true. I have a feeling I will heard me about this tomorrow.

tl;dr College announced the senior class trip friday, I don't want to go, sign ups are monday, limited amount of people can go on the trip (about 60), roommates probably won't have enough time to find a fourth hotel mate, they are going to be mad at me because they can't go.