Different beliefs

Tammy
Ok so just talking to the bf re cheating. 
Long story short his brother has cheated on his gf cos he's not happy in the relationship she's a gold digger and is expecting him to support her whilst she's at uni the next 4 yrs. 
I said to my bf I don't care how bad she is or what she's doing wrong even if she's cheating or whatever he doesn't have the right to cheat on her, he should be the bigger person and walk away. 
My bf has cheated on exs in the past etc and after I explained that I thought two wrongs don't make a right and his brother should have just ended it my bf goes I my god I can't believe I'm hearing this are you for real and hung up on me and is now ignoring me...
Making me re think our whole relationship...if he believes that it's ok to cheat when the going gets tough on not sure he's the man I wanna spend my life with 
What do you all think??