Just curious.
Does the Buddha proclaim as sin the following?
Buddhism is not an Abrahamic religion. It is often an error to assume that concepts from the Abrahamic religions transfer across into Buddhism without checking first. This is a case in point, Buddhism does not have the concept of 'sin'. Instead it has wise actions and unwise actions. Wise actions produce happiness, while unwise actions produce suffering.
Mind precedes all conditions,
mind is their chief, they are mind-made.
If you speak or act with an evil mind then suffering will follow you,
as the wheel follows the draught ox.
Mind precedes all conditions,
mind is their chief, they are mind-made.
If you speak or act with a pure mind then happiness will follow you,
as a shadow that never leaves.
– Dhammapada 1:1-2
As the text indicates, the state of mind behind the action is important.
As with Christianity you can find opinions either way in Buddhism. Where allowed, it must be between consenting adults and any promises to spouses, such as remaining faithful, must be kept.
(2) sex before marriage (aka fornication) Yes or No
Buddhism did not originally have an equivalent of marriage, it recognised whatever local customs were already in place. Hence before or after marriage is not a relevant question in Buddhism. Again, consenting adults are required, and any promises to spouses must be kept.
(3) hard core pornography Yes or No
A matter of motive. Showing a picture of Michelangelo's David (sans fig leaf) may be considered pornography, or it may not. For an art appreciation class it might not; in other circumstances it might be. A medical textbook may have very explicit images of human surface anatomy, but would not generally be considered pornographic.
Alternatively, a photo of a woman's feet might be pornographic to a foot fetishist, but not to anyone else. The definition of what is, and what is not, pornography is slippery.