Is there any excuse for cheating?

I'm in love with an American. We are thinking of getting married, but there are problems that we cannot overcome. I am the daughter of a Turkish family and our perspective on issues such as jealousy and cheating is very different. He's a lecher and it hurts me that he's doing this around me. Do all Americans see this as normal? Because every time I tell him that it upsets me that he did this, he repeats the same thing. He has more pictures of other girls on his phone than mine. I've read the Bible as well, but it tells me that such things are not wrong with Christianity. I used to go to church when I was in Turkey, but no one thought it was true. He thinks I'm shallow-minded. Is he right?

Is there any excuse for cheating?
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