Consider the following (probably fictional) scenario, described in detail by pop culture writer Chuck Klosterman1 and paraphrased here: Jack and Jane are in a happy romantic relationship for 2 years. One day Jack receives an invitation from another woman living in his building to watch her masturbate in her apartment (with absolutely no physical contact and no emotional intimacy). Intrigued, he goes to her apartment to watch her masturbate, then returns to his room and goes to sleep. Jack believes this episode to be weird/strange, but not unethical. He innocently mentions it to Jane, who upon hearing this, becomes extremely upset and ends the relationship, cutting off all contact with Jack.
What do you think about this situation? Did Jack do anything unethical? Is accepting an invitation to watch someone masturbate (while in a relationship with someone else) a moral violation?