Watching Porn In Public Versus Private
Also If a married couples watches adult movies/pornographic movies together then the whole base of sadde zariya on this matter will contain less value. Cause sadde zariya on this matter is refering to the chances of being addicted to it and then commit zina but if a married couple does this then there will be no chance of zina taking place.
As Umer sb said, looking at private parts of people other than their spouses comes under fawahish. Hence, it will be considered haram. Read verses in Surah Aaraf 25-29. It will become more clear Also watch https://youtu.be/b6ykH8zeMjU