Relationship HELP NEEDED PLEASE!!!?

Hi ill make this as short as possible so i dont bore yous, well me and my partner have been together a year and a half now and i was on his laptop the other day and noticed hes been using msn, myspace and that plentyoffish to talk to other les when i pulled him about it he just said he was doing it for a laugh winding them up ( some of the pepole he was talking to were about 60!) hes 32 and he said that every man does it and its not as though hes cheating its just a laugh. Anyway i took this quite badly at 1st and moved out but i really miss him and love him. he has asked me to go back and says he has deleted the profies off myspace and plentyoffish and closed down his msn account (which he has ive checked) he said he realises what he has done was wrong and apologised and said he would neva do it again. do i believe him and go back or not??