Where is the line of cheating in a relationship in regards to "sexy texting" with "friends"?

My boyfriend revealed to me a topless photo his friend had sent him, to which he replied, yeah can I have another. I consider this cheating. Yes he revealed it to me, but he also lied to me for a month straight, and I had though our relationship better than ever during that time. Is this a minor transgression? He had always said he was so appalled by his friends overt sexual behavior. Soon as he got the chance, he played right in. Where is the line with cheating concerning the internet, texting, dating apps. Wld luv to hear some thoughts.