No and yes.

I believe that for some people serial monogamy works, for others it don't. I believe it is none of my buisness who or how many people you sleep with as long as a) it is all consensual b)all parties involved are mature enough and c) all parties involved are aware of what is going on. If you are dating one person, but at the same time sleeping with someone else and neighter of the two others know, then you are cheating on them (yes you are cheating on both of them). However if they know and have accepted it, then you are in an open relationship, which is different and alright.