Let's look at the words.
Marriage is a type of yoke. When Matthew and Mark write about man and woman “joined” together by God in marriage (Matthew 19:6; Mark 10:9), they use a Greek word with “yoke” as its root. This root is the same as the one used in the 2 Corinthians 6 passage. In other words, the verbs in Matthew 19, Mark 10, and 2 Corinthians 6 are all etymologically related. God has “yoked” man and woman together into a one flesh relationship.
Definitions for suzeugnumi (literally “yoke”).
"Yoke" was the common term used for marriage in the first century. The husband and bride were yoked together for life.“So then, they are no longer two but one flesh.
Therefore what God has joined [yoked] together,
let not man separate”
Matthew 19:6“Therefore what God has joined [yoked] together,
let not man separate.”
Mark 10:9
Definitions for heterozygeo, which is translated “unequally yoked”:
“To be wrongly or poorly matched in an association - ‘to be mismatched, to be wrongly matched.’ ‘do not be wrongly matched with unbelievers’ 2 Cor 6.14. It is often necessary to indicate somewhat more precisely the manner in which one may be wrongly matched with others. Accordingly, one can translate 2 Cor 6.14 as ‘do not attempt to work together with those who are unbelievers’ or ‘do not become partners with those who do not believe.’ Louw, and Nida, Greek-English Lexicon of the New Testament based on Semantic Domains.
This word derives from heterozygos. “unequally yoked” (e.g., the yoking of an ass and ox). It occurs for the first time in 2 Cor. 6:14, where Paul uses it to describe the abnormal situation that results when Christians fail to maintain their distinction from unbelievers.
Kittel and Friedrich, Editors, The Theological Dictionary of the New Testament.
To be “unequally yoked” would naturally be seen as a joining of two in marriage who do not belong together.
Yes, this is the most popular belief. And the explanation makes good sense, and you may be 100% right, but there is still the possibility that Paul meant what I heard the pastor say.