I am not consenting in my recognition of marriage existing between any combinations of parties beyond one genetic male and one genetic female. Gay marriage harms me by forcing me to accord it the same treatment. That is a curtailment of my individual rights to liberty and property.

As far as the harm caveat, in no way, shape or form could an unborn fetus be considered to be capable of giving consent. Abortion is therefore wrong. The various arguments used to except the fetus from human rights have also been used regarding racial minorities and mentally or physically handicapped individuals or classes, and have also been generally rejected by moral reasoning.

Gun possession and use harms no one, except under existing laws against assault and murder. So long as assault, murder, manslaughter, robbery or threats to life, liberty or property are not done, there is nothing wrong with a person owning, carrying or using a firearm by your definition.

Taxes, on the other hand, do harm to adults by confiscating their property. Direct taxes were unconstitutional in the US for over a century for the very reasons you give.