1)  You don't know what people do behind closed doors.
2)  It is not only gays who engage in what you call "sodomy"; heterosexuals also do so.
3)  If you think that an action is sinful, you ought to refrain from it.  However, since we do not live in a theocracy, others are free to define sin or inappropriate behavior as they wish, as long as they are not harming others and not violating civil laws.  
4)  Because we do not live in a theocracy, and because same-sex marriage is lawful, it is valid marriage.