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. It does not mean Christians can not associate with non-christians. It only applies to the binding relationships with non-christians. Even Jesus associated with non-christians.