Would it be a sin for a straight man to marry a biological woman who identifies as a gay man?

No. God made them male and female. Men and women naturally get married. However, if the gay person is having gay sex outside of marriage that is a HUGE problem.

If you are inquiring about the Biblical definition of 'sin,' this topic is much easier to discern than most people make it. 'Sin' is any behavior or activity that results in a needless human death, (James 1:15).

This can happen immediately as a direct result of the activities, interimly by way of example, or ultimately through a larger social ripple effect. The Bible does not categorize people using social generalizations. It forbids the use of these as something called 'kategoria,' (Greek: 'accusatory divisions'), and identifies them as the technique used by evil to more easily manipulate, threaten, judge or sacrifice otherwise unique individual persons as part of some cause presumed to be 'greater' than their own lives or families. This is why Jesus forbids Christians from swearing by oaths to any cause for any reason, (Matthew 5:34-37). It is most often by oath-taking that we surrender our unique individual identity to be part of a group like 'straight' or 'gay.' That is why neither is ever used in holy scripture.

The use of these social identifiers ALWAYS results in someone being considered 'expendable' within that demographic viewpoint. That makes ALL forms of their use 'sin,' including your example.

Yes. God made male male and female female. You don't get to pick and choose your gender at whim.

Yep.

no