They will never be able to pull equally. The product of yoking them together will result in very crooked rows in the field and a tired ox and donkey. Paul used this analogy of yoking unequal animals together to show unbelievers and believers they should not be yoked together. Paul here uses the word yoking to refer to a binding relationship. In this kind relationship, people are either bound to each other for life or for a period of time.
Marriage is no doubt a binding relationship.
Dating, leading up to marriage can also be considered a binding relationship. Another relationship is business partnerships. Partnerships are binding to both parties for a length of time. Any relationship that binds a non-christian to a Christian in any way can be considered unequally yoking. Some people take this unequally yoking definition to extremes.