Wth...all liesssssssssssssss?

Okay, so I've been dating someone for the last year, but we've been in a breakup lingo for the past two months. Lately though, things have been pretty good, or atleast I thought. I just found out that he had with someone over spring break and that he's been talking to another girl for about a month. The girl that he has been "talking" with is from out of town and is visiting him right now, after I just left his house! He just told me last night that there were no other girls and that I was the only one he was sleeping with, however, the source that I found it out from is very reliable, but when I asked him about it he denied it all and said, "we aren't together anyway so why does it matter?" I understand that we might not have the boyfriend/girlfriend title right now, but we still have a relationship....period. If he isn't doing anything wrong and feels no obligation or loyalty to me anymore, why does he need to lie about what he's done/doing? All that I asked for is honesty.