Be up front about it. A lot of times, friends that are in relationships, whether or not they are happy couples, tend to completely forget what it feels like to be single and unhappy about it.

Of course I just assumed that you were unhappy about being single but, if that's not the case, you honestly still need to just tell them you're not part of their relationship and they need to stop dragging you into it like you were. Will some of them get mad? Maybe, but if they're really your friends they won't stay mad, and you'll save yourself a lot of headache.