Yes, there is no excuse for straying while married.
Yes, the unfaithful partner is solely to blame in instances of infidelity. While relationship problems may be the result of the actions of two people, steps should be taken in order to either rectify the situation or to end the relationship. Deciding to cheat is a personal decision, and one spouse cannot drive the other to cheat.
The Cheater is at Fault
Regardless of whether your relationship is bad, terrible, co-dependent or stupid, the cheater is always at fault. Two wrongs don't make a right and blaming infidelity on the person who did not cheat is always a good sign that the cheater is a morally reprehensible individual who needs to not be in that relationship anymore.
Yes, I think an unfaithful partner is solely to blame for his or her infidelity.
Overall while the relationship may be having problems and that will push the person to commit infidelity I think that it is ultimately the decision of the person to decide if he or she wants to remain faithful or not, I don't think it's right or fair to say that the other person should be blamed.
No, not always
People are not faithful to their partners for a bunch of different reasons. One may be that they are not getting attention or feeling respected. It doesn't always mean that it is solely that person's fault. I think if someone is cheating then a lot of times it means something is wrong in the relationship.