I noticed you wrote "doing porn" and not "watching porn".Makes sense to me. If you don't have high enough morals to keep you from doing porn, then you also don't have high enough morals to realize that marriage is the union of a man and a woman.
Are you trying to find some wiggle room you don't want us to notice?
the social institution under which a man and woman establish their decision to live as husband and wife by legal commitments, religious ceremonies, etc. Antonyms: separation.
a similar institution involving partners of the same gender: gay marriage. Antonyms: separation.
the state, condition, or relationship of being married; wedlock: a happy marriage. Synonyms: matrimony. Antonyms: single life, bachelorhood, spinsterhood, singleness; separation.
the legal or religious ceremony that formalizes the decision of two people to live as a married couple, including the accompanying social festivities: to officiate at a marriage. Synonyms: nuptials, marriage ceremony, wedding. Antonyms: divorce, annulment.
a relationship in which two people have pledged themselves to each other in the manner of a husband and wife, without legal sanction: trial marriage.
any close or intimate association or union: the marriage of words and music in a hit song. Synonyms: blend, merger, unity, oneness; alliance, confederation. Antonyms: separation, division, disunion, schism.
Pay attention to definition #5. The word "marriage" includes more than just your man/woman ideal. It applies to the marrying of flavors in a recipe, the marrying of two ideas and much more.
You can define marriage for yourself but you don't stop there. Instead you want to define the world for everyone else and force your lifestyle and values on everyone else. And that's exactly what you complain about people doing to you.