Shirk is a grave matter. We cannot trivially call things Shirk. And there is no reason or evidence to consider watching videos or movies Shirk. But, that does not mean it is good or allowed to do. Drinking alcohol doesn't make you a Mushrik, but it is still haraam.
First of all, about pornography, that is definitely forbidden:
Tell the believing men to lower their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do. (24:30)
Do not give a second look, Ali, (because) while you are not to blame for the first, you have no right to the second. (Abu Dawud)
If we can't even look at the opposite gender directly... it is definitely forbidden to watch pornography.
Watching youtube videos, movies, and TV shows does not make you a Mushrik or Kafir, but there is definitely evidence that it is a sin to watch useless things:
In gardens they will ask one another (40) Concerning the guilty: (41) What hath brought you to this burning? (42) They will answer: We were not of those who prayed (43) Nor did we feed the wretched. (44) We used to speak uselessly with those who spoke uselessly, (45) And we used to deny the Day of Judgment, (46) Till the Inevitable came unto us. (47) (Al-Mudathhir)
Wasting time discussing or listening to uselessness is mentioned along with everything else that are clearly sins.
And it has already been revealed to you in the Book (this Quran) that when you hear the Verses of Allah being denied and mocked at, then sit not with them, until they engage in a talk other than that; (but if you stayed with them) certainly in that case you would be like them. Surely, Allah will collect the hypocrites and disbelievers all together in Hell, (4:140)
Many shows and movies openly deny the God. And even worse, many openly mock him. How can a person in good faith give their time up to watch such videos (except to actively correct them)? It would be a sin as it is against Allah's commands.
Then which of the Graces of your Lord (O man!) will you doubt (55) This (Muhammad SAW) is a warner (Messenger) of the (series of) warners (Messengers) (56) The Day of Resurrection draws near, (57) None besides Allah can avert it. (58) Do you then wonder at this recitation (the Qur'ân)? (59) And you laugh at it and weep not, (60) Wasting your (precious) lifetime in pastime and amusements. (61) So fall you down in prostration to Allâh, and worship Him (Alone). (62)
Wasting time is no good, when you can be doing useful things. As Allah himself commands you in the above Ayah, worship Allah more if you have time. Don't waste it on dubious activities.
