At first I thought you should talk to the new guy and notify him that you'll be breaking up with him if he continues to give you the cold shoulder. But then when I read about how severely the situation was affecting you, I changed my mind and now I think you should just break up with him, period. You need to remove this guy from your life, even if it's really painful (in the short term) to do so.
There isn't a "typical" reason for a guy doing what this new guy has did. His behavior isn't typical behavior (poly or otherwise). I suppose it could be as simple as, he met someone else he was more interested in, and being an ADD type or something, he then lost all interest in you. Though like Marcus said, that's nothing but sheer guesswork on my part and as such probably isn't worth a lot. The only way to really find out is to ask the guy directly (and hope he answers honestly).
GalaGirl is right in saying that if you don't want to fall in love, you have to refuse to go along with any falling-in-love types of interactions. If a guy approaches you with sex and/or fun, say yes. If he starts texting/chatting you all the time and telling you he's smitten with you, end the relationship right there. Make sure you warn all guys you get involved with that going beyond sex and fun will result in your immediate withdrawal from them.
If you are open to the possibility of falling in love, then you have to be open to its inherent risks. Emotional vulnerability means the possibility of being badly hurt. It's only worth it if you think it's worth it. Worth the eventual romance that turns out to be the real deal for you. That's the prize you seek when falling in love, and the price for finding it might be a broken heart or three.
You should probably take some time to think about what you really want, what your boundaries are and how you will stick to them. And sure, "sex and fun" are more swing/open relationship type things. "Falling in love" is more of a polyamory type thing. Not that the terminology is the priority here; it's more important for you to figure out what you want and figure out how to stick to it.
In any case, remove this new guy from your life as soon as you can. He's not providing sex, fun, or love. Probably not too good of a guy based on how he's acted; it's actually lucky for you to get rid of him.
Love means never having to say, "Put down that meat cleaver!"