Christians in Marriage - Unequally Yoked

"Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? And what concord hath Christ with Belial? or what part hath he that believeth with an infidel? And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people." (2 Corinthians 6:14-16, King James Version)

What does it mean "Unequally Yoked"?
"Thou shalt not plow with an ox and an ass together." (Deut. 22:10 KJV)
This passage from the Old Testament reads in the NIV: "Do not plow with an ox and a donkey yoked together."
They would not be of equal strength, disposition and ability, thus the plowing of the field would be made far more difficult than was necessary. It's possible one would fall under the unequal burden. To offer an even more dramatic illustration, if a large horse and a small goat are yoked together, it is highly unlikely that the farmer will be able to plow a straight furrow in his field. His team is clearly "unequally yoked," thus one member will easily and inevitably overpower the other, with negative results!