Interesting dilemma and one I am familiar with... you can have a look at my blog to see details. Around Christmas (2010) was when I struggled with a similar issue.
For me I had known the person for two years and we wanted to have sex in our relationship where we had not before. We had a non-sexual relationship before but considered each other partners.
The similarity here is that you have a closed poly-fi relationship that you now want to open due to your desire to have casual sex. I would wonder if it is worth it. For me it wasn't. There would be a chance that I would lose everything I had worked for and I wasn't willing to do that for this person. At least not right now. I am very happy with that decision now that I can see past my desire a bit more.
I would be asking myself what is more important, your living situation and the support of your loves? Or casual sex with someone you are just getting to know and have NRE with...?
If the answer is that it is worth looking into and you want to right now, rather than waiting to see if the desire dies down or it becomes something more, then start talking. It seems to me that the best place to start would be to let the others know about your desires and subsequent discussions with this person. I was wondering why you hadn't already actually as usually, in the spirit of open and honest communication, these kind of big feelings are out there already by the time it gets to discussing getting together to fuck. Still, no time like now to get talking.
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