There was a day when former President Jimmy Carter claimed allegiance to the principles of Christianity. It's a shame that political expediency and posturing has come to replace that allegiance to truth as his first priority:
Watch this video.
Dr. Albert Mohler has explained the conservative position on this issue quite well:
The conservative view is that all sexual deviance - homosexuality, polyamory, adultery, bestiality, incest - violates the natural order. Families depend on moral structure: Mom, Dad, kids. When you confound that structure - when Dad sleeps with a man, Dad sleeps with another woman, or Mom sleeps with Grandpa - the family falls apart. Kids need clear roles and relationships. Without this, they get disoriented. Mess with the family, and you mess up the kids.
Carter is smart enough to know that. Carter was once grounded enough Biblically to know that. What a shame he has taken the path he has taken.