What will u do if u found out that ur lover flirting somenoe else?

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Im in a relationship right now,we've been together for four long years and not have plans yet for a life time commitment, but in my heart i know he's do one i want to grow old with.. Until one day, he accidentally forward a textmessage that was not really meant to me... I found out base on the text message that he's actually flirting someone else and i get mad right that minute, i confront him but he keeps denying he's flirting some else. He said its just a friendly conversation.. What should i do? Will i give him my trust for the second time around or just give up the relationship?