Perhaps a more apt word would be hypocritical.

Also, if a relationship involving kids collapses (for whatever reason), there is no guarantee that the kid's life is definitely gonna be more fucked up than if the parents stayed together. If a relationship is not meant to be, then its not meant to be and one way or another it will collapse. Thats life. I am not condoning a guy having sex with a woman who has a fella and kid, not one bit, but by walking away from this you're not ensuring the defenceless are going to remain unharmed, as if the woman wants to sleep with someone else, then she will.

With reference to the OP - if bcb ends up shagging this bird then the person I would feel most for is the guy thats been cheated on (this is presuming he has been faithful in the relationship). Not only has he been fucked over, he then has a difficult decision to make - stay with the girl despite the fact he won't be able to truly trust her again, or finish the relationship and become a part-time parent in the process, as his chances of getting sole custody would be somewhere between zero and fuck-all.