Obligatory infidelity is getting involved with someone else out of a sense of obligation. It often occurs in a career advancement type situation as well, where you feel if you don't get involved with your boss, you would get fired.
It also occurs in situations where there's some kind of role disparity, where someone in some way is submissive, and the other person is more dominant. For instance, Arnold Schwarzenegger's maid might have gotten involved with him from obligatory infidelity perspective. Of course, it was also an opportunistic infidelity as well.
These kind of cases of infidelity cannot just be put in one category. They in many ways have elements of different types of infidelities. Someone may have gotten involved in an obligatory infidelity, but then developed romantic feelings as well, and then it becomes a romantic infidelity as well.