Am I being used?

There is a supposedly straight girl who has a boyfriend that I hang out with just for . she calls me every now and then, we go out but we always end up sleeping together and she always leaves and I don't hear from her until she contacts me and we wind up having again. It's a no-strings attached situation. I asked a question earlier and some people suggested I was being used. I'm not sure though cuz I quite like it. It's a liberating and pionate experience. By the time we meet each other again we're so hungry for each other we can't keep our hands off one another. The only thing I'm not comfortable with is that she's never told me she has a boyfriend I just know it through other friends. Am I really being used and I don't see it?