F.R.I.E.N.D.S

I thought they where supposed to be fun?

Last weekend I went to a friends birthday dinner which included family and friends. I knew a couple of people there so it was ok as it can be. The birthday girl was planning on going out afterwards with all of her friends. Sometimes I feel bad for not wanting to go out clubbing afters, however, I think again and I shouldn't feel bad! I should be able to have a choice of going or not going without any hesitations, right? Well in this case wrong. Some of her friends left and she was ok - happy they came, happy they left - at what ever time. One of my other friends asked if I was going out with them after the dinner party was over. She made a huge deal about me not going out. She kept pestering me and asking why I wasn't, I understand that she may want me to go out with them but stop at a certain point... I felt really embarrassed because of the big deal she made. Why can't I just say what I want to do? It didn't stop there.
She called over the birthday girl and told her I was not coming out. At this stage I didn't have the chance to tell her myself so I felt more guilty about my friend telling the birthday girl. The birthday girl says:
"Oh, are you going to sleep with "(a male friend") instead?" and storms off!

..excuse me?
What makes a person say that to someone?
Especially since I have never slept with this friend of mine...And I don't talk about my personal life in detail to her..
I didn't argue with her or confront her because a)it was her night and b) i just let things like this blow over, i don't make things into a big deal - as you can tell.

but why do that? i want to be able to make my own decisions without feeling awkward.