Is it ecthical to come to the competitor party?

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My ex boss who noe become our biggest competitor throw a personal party, he invited all of his ex employee to come to the party. Although we all know that this party is personal, but my current boss try in many ways to prevent us to come to the party. He persuade us not to come and even give us a minor threat. The party already done, but give me some questions. Is it so un ethical to come to the competitor party even though a persoanl party? Is it right for my current boss to prevent us to come?