Cheating is always wrong. There isn't anything that can validate that behavior. Nor should you validate that behavior in your friends or family. Nevertheless, if a cheater isn't willing to admit they were in the wrong, they'll do whatever they can to defend their behavior.
I (22M) am the youngest of five siblings, but my issue is with my brother Matt (32M). I'm gay, and people tell me I'm being homophobic. I came out as gay at 16 because I started a relationship with my current boyfriend. My whole family was so supportive, my parents, my other brother, and my sisters, but my brother Matt, for some reason, didn't accept me and distanced himself from me. Matt would ignore me at family gatherings and speak the bare minimum.
Matt was married to Marina (29F), and they have a five years old adorable boy that my family adores. A year ago, my brother was caught cheating on Marina with a man, and he ended up coming out and divorcing Marina. He dumped Marina right after that, and my family supported him and welcomed the AP as if nothing had happened. The affair had been going on for years before Marina found out, Marina was deeply hurt, and I felt sorry for her. I've known Marina since I was around eight years.