So do you feel like it's a problem or don’t you? Do you think you're sabotaging relationships that you would really prefer to continue, or are you reasonably happy with short-term arrangements for the moment?
Personally, I don't think there's anything wrong with keeping things light if that's where your life is right now. You may want to make sure your partners are on board with this, so as to minimize disappointment all around, but if commitment isn't your thing at the moment then go with what works. 27 is pretty young, all things considered (though, granted, still older than me

), and it seems like you have time to sort out what you're looking for. The only problem I can see is that it may be a little more difficult to find people in the age group you seem to favor who are similarly inclined to keep things casual, but you don't seem to have much trouble there.
So basically, if this feels like a problem to you then maybe you should take a good look at whatever impulse is making you withdraw from these relationships. But if you're happy and satisfied with the relationships you have, however brief, then just go with it and try to find partners who are looking for the same thing.