Hmmm, cyclinggal, you are full of inquisitiveness today LOL.

For me, it's adultery or lying/cheating in any way, for any reason.

Also if that person lacked clear, defined boundaries in their lives would be a deal breaker for me too. (Bad boundaries for me means things such as needy behavior, passive-aggressiveness, insecurity issues, refusal to understand how their behavior affected others, etc.)