ANSWER: Is it a sin to move one's feet in a rhythmic fashion (dancing) either as entertainment or as part of worshipped God? The KJV word for dancing (or variation thereof) is found only twenty-one times in the Bible. It is important to realize that the worship of God in ancient times was considerably different than it is today.
What we, in our conservative ways, might think is wrong would have been quite acceptable to some of our Old Testament friends who worshipped him with unabashed exuberance and, yes, dancing.
Dancing was not uncommon among the ancient Israelites and was employed on various occasions. Miriam, the sister of Moses, and other Israelite women, celebrated the total destruction of Pharaoh's army in the Red Sea by joyously moving their feet (Exodus 15:20 - 21).
King David, when the Ark of the Covenant was being brought to Jerusalem for the first time, celebrated by dancing around it during its journey (2Samuel 6:12 - 14). Such joyous behavior was common during God's annual Feast Days (Judges 21:19 - 21), in public triumphs (Judges 11:34), and during anytime that required rejoicing (Psalms 30:11, Jeremiah 31:4, 13).
Dancing was also used to honor and praise the Eternal during religious services (Psalm 149:3, see also Psalm 150:4).
King Solomon, the wisest man who ever lived, stated that there was a time for dancing (Ecclesiastes 3:4)! Clearly, such behavior is an acceptable part of worshipping the true God.
There is no direct statement in the Bible forbidding married couples from dancing or calling the act, in all circumstance, as being a sin. The acceptability to perform this activity for all others, however, depends on the situation.
The Biblical Illustrator Commentary states in regard to the appropriateness of this act, "Dancing in itself, as it is a set, regular harmonious motion of the body, cannot be unlawful, more than walking or running. Circumstances may make it sinful . . ." (Biblical Illustrator - the Gospels, commentary on Matthew 14:1 - 11).
Jesus taught that lust, which is the strong desire to have sex with someone who is not one's mate, is wrong and a sin. Although his teaching discusses lust of men toward women, it is meant to apply to ANY human toward another person (Matthew 5:28).
Sexually suggestive dancing (performed by a person to "entertain" others, by people who are dating, etc.) is not appropriate. Such behavior can tempt others to commit further sin beyond any lust they create in their hearts. That said, Scripture permits a person, either by themselves or with others (e.g. singles who are dating) to dance so long as their motive is not to entice others to disobey God.