Well, let me put it this way.
If you were dating one guy, and caught him seeing another woman, he'd be hunting for certain parts of his anatomy for the next six months - right?
Men are far more jealous in relationships than women.
So no. Stop being a tart and pick one. Preferably the right one.
Its not cheating to date two or more people at the same time as long as neither relationship is exclusive. If a guy asks "are you seeing other people" then obviously be honest about it. Its not against dating rules. If men get jealous then they have the option of asking the girl to become exclusive boyfriend/girlfriend. That's the price you pay for waiting to become exclusive, you leave the person you're dating open to dating other people. If that converstation hasn't happened, then neither party has territorial rights to the other.
I for one haven't really dated multiple people long term, but if I'm dating someone and I'm not quite sure about them and we're like 3 dates in, no discussion of exclusivty, and another guy asks me out then I'll set up a date with him. That's what dating is, and that's why you have a conversation about being exclusive. It can't just be assumed.