I pretty much agree with t4u. If the reason this guy left you was to pursue another girl, and now he wants to come back because things didn't work out, it's hard to imagine why you would say "yes". You don't want to be this guy's safety net, right? If you take him back, what's to stop him from chasing the next pretty girl that catches his eye? Do you think you will ever be able to trust this guy -- really trust this guy -- again? And what kind of relationship will you have with this guy, going forward, if you can't really trust him?
Well, I do believe in second chances in life, people do make mistakes. The one mistake I don't give a second chance on is being untrue. This guy wants back, because he thinks you'll take him back. Don't lose your self respect over this loser. If he couldn't love you faithfully then, it's a good chance he can't now. Your just backup to him until the next woman comes along and you get dumped for another, again. Don't do it, your too good for that. Love shouldn't be wasted on people who can't give it the respect it deserves.
You should probably say good riddance. If he was willing to leave your fabulousness then he doesn't deserve you. However if you want to give him another chance you can. Every one deserves a second chance. But if you are going to make it work a second time around then some serious conversation/communication/talking whatever, needs to happen and the root of the problem ie: the reason he left you for this chick, needs to be discovered and taken care of and the serious trust issues that you probably have with him now will need to be addressed. However if you don't feel like this problem can be taken care of then you have every right to shut him down. Honestly, if my boyfriend left me for some other chick and then tried to get back with me I would not be having any of that.
Ew. My x boyfriend did this to me. However, I'm letting him rot in his guilt. I won't go back to him if he paid me a million dollars. Break-ups happen for a reason obviously yours is right in front of your face. He left you for someone else then wants you back. Let him go he needs to grow up and stop playing you. You're worth more than giving a sad piece of human your heart again. He'll respect you more for NOT taking him back. I know this because it has happened to me. It will drive him crazy.