If there is no legislative momentum to include incestuous marriage, it won't be included within the definition. Homosexuality has been decriminalised (if it ever was a criminal offence) in those nations where "gay" marriage is now on the agenda.
Do you think incest has sufficient representative support to be decriminalised so that the world slips down the slope you envisage?
When thou goest forth to war against thine enemies, and the LORD thy God hath delivered them into thine hands, and thou hast taken them captive, And seest among the captives a beautiful woman, and hast a desire unto her, that thou wouldest have her to thy wife; Then thou shalt bring her home to thine house, and she shall shave her head, and pare her nails Deuteronomy. If they accept your terms and open the gates to you, then all the people inside will serve you in forced labor.
You may enjoy the spoils of your enemies that the LORD your God has given you. I think Gravity's saying that a marriage between two men or two women is no less perverted and unnatural than that between a parent and his/her child. There is one key difference though, and that's if a gay couple want a child, there isn't the same risk of inbreeding depression (unless one half of the gay couple is genetically related to the donor of the egg/sperm). Before you say "same old shit" as if the fight for "marriage equality" has been going on for centuries, truth be told it's only a relatively recent phenomena that everyone has been jumping on this "marriage equality" bandwagon.
But you may keep for yourselves all the women, children, livestock, and other plunder.
You may enjoy the spoils of your enemies that the LORD your God has given you. As you approach a town to attack it, first offer its people terms for peace.
On 'a peaceful, moderate Muslim is just one who hasn't killed anybody yet' ...