A crime is an act that causes objective harm.* A sin is a violation of a sincerely held religious belief. A taboo is similar to a sin insofar as it involves violation of a societal norm. Sins and taboos may be crimes as well, but their criminal aspect is independent of their status as a sin or taboo. (And, obviously, a sin may also be a taboo and vice versa.)
It is a sin for an Orthodox Jew to eat pork. It is not a sin for a Baptist to eat pork. A Baptist would not make a very good Orthodox Jew if she was unwilling to change her beliefs and actions.
Likewise, no matter how righteous an Orthodox Jew may be, he would still be committing a sin in the Hindu or Jain religions if he ate beef, no matter how kosher the preparation.
To many people, committing a homosexual act is a sin. To them, it most certainly is: They should refrain from committing homosexual acts.
But to other people, committing a homosexual act is not a sin. By definition, this is just as legally valid an expression of a religious belief as those who hold it is a sin.
The next question comes as to whether a homosexual act is a crime. Again, assuming informed consent among all involved / interested parties, an action between / among homosexuals is no different from an identical action between / among heterosexuals. Without getting too explicit about it, there is no action homosexuals can perform that cannot be performed by heterosexuals, by definition there is one action that is always heterosexual even if performed by homosexuals (of different genders, of course).
If there is no difference in non-criminal actions between those performed by consenting informed heterosexuals and those performed by consenting informed homosexuals, then there should be no difference in civil laws or tax codes.
One may still hold a lawful action is a sin for a particular religious group while at the same time acknowledging those who perform such an act are entitled to the same legal protections one enjoys. An Orthodox Jew can find a Baptist's pork eating to be utterly repellent without denying the humanity and civil rights of the Baptist.
* My full definition: "A crime is a discrete act or series of actions that either causes or is capable of causing objective physical harm or property loss to a human being without their prior informed consent." e.g., an entrepreneur who informs investors it is possible to lose their entire investment has not committed a crime if his business fails; an entrepreneur who promises such an investment is guaranteed when it is not has committed a crime even if it succeeds. A football player who cripples an opponent with a clean tackle has not committed a crime because his opponent was informed there would be rough physical contact while the ball was in play and consented to same, a player who blindsides an opponent after the whistle is blown has committed a crime.