Sex parties are actually quite common and I have been to a few myself.Just because I am a Christian does not mean I am a prude.
A date would work but not with relationship as a goal, to be honest here.
I do not want to relive those old college days where I went to frat parties and wanted to see how much alcohol I can hold. Since you are older and more sophisticated, you can do older and more sophisticated things. You can also chose to cater the event and have fancy cocktails with fancy umbrellas in them.
So why not have a luncheon where you invite all of your girlfriends over for the day! Ask you girlfriends to dress up in their latest sun dresses and really make a go out of it.
The point is that you did not drink legally so why not take advantage.
Have each of your girlfriend bring their favorite wine over and have a wine tasting party.