Problem with keeping stories short?

I'm new at this, haven't published anything yet. My main issue is that every time I start trying to write a short story (5k) it ends up turning into some huge thing, and I get all these ideas and new storylines and it gets way out of control into something I never finish.

How on earth do I keep things short and to the point?? Halp plz.

Edit: basically my erotica attempts turn into wannabe erotica romance. I want to write that genre eventually but for now I want to get some erotica shorts actually completed and published.