I would suggest going to their pref party, and then if you really don't want to be a part of the sorority, then don't pref them. I can tell that you have thought long and hard about this, and I would continue thinking about it. Talk to your recruitment counselors and get their advice about COB.
Bottom line.... You don't have to do anything you don't really want to do. If you honestly will not be happy there, then you don't have to join. Do what is in your heart. I will add that I was not the most excited about my option when I went through pref, or even bid day. But, I did grow to love it and I found my home and my sisters.
I am SO sorry that things have not worked out the way you were hoping.
Trust me, there are alot of ladies on here who have been in your shoes, and I promise things will work out the right way. You do always have next year if that is what you want to do.
Looking on the SHSU website, it looks like at least one of the chapters does spring rush. Have you thought about that? It definately might be something worth considering.