The context of A yoke conveys the idea of two bulls joined to the task of pulling a plow, but they are not well matched.
They are unable to pull the plow in a straight line. This is what it looks like when one partner follows Christ, and the other does not.
A believer and a nonbeliever united in marriage is not therefore a good fit.
However, I for one think that Paul was talking more of a marriage relationship but Paul doesn't limit it to just the union of marriage.
The very nature of the Christian demands that he be separated from that which is unholy. When a saved person marries an unsaved partner, it sets up an impossible situation; and the same thing applies to business partnerships and religious “fellowship.”