Solipsism is subjective idealism. The notion that an individual can determine their own gender is substituting subjective determination for objective determination.
Social rules are just as much an objective reality for individuals (subjects ) as the physical world is . To claim that one can as an individual change the customs and culture , objective reality, is solipsism ; with respect to SOCIAL objective reality. An individual cannot change the rules of a culture any more than they can change the laws of thermodynamics . Society is an objective reality for individuals; that's materialism.