God's law is summarized in the Ten Commandments, so "Thou shalt not commit adultery," includes homosexual behavior, as well as all other forms of sexual immorality. The reason the Church focuses on homosexuality more than strict adultery, is that adultery is generally looked down upon in our culture. Almost anywhere you go you can see a sign or a bumper sticker that says something along the line of "Gay ally," but you'd have a much harder time finding any "Cheater ally" signs.