"See? See? He wants it for himself" was the gleeful comment of Gollum as he succeeded in turning Frodo against Sam near the end of the third LOTR movie. Sam was a threat to Gollum because he knew Gollum intended to kill them both, so the best way to get rid of him was to have Frodo order him away. And the only way to do that was to use the power of the ring (a symbol of the power of sin) to poison Frodo's mind against Sam.
That’s why God tells the people of Israel not to marry into the families of the land (Deuteronomy 7:3-4). It was a strong prohibition against marrying their sons or daughters, because God knew where it would lead: a rejection of Him. Close connections to pagan idolaters would poison their minds and lead them away from God, just like Gollum was able to poison Frodo's mind and turn him against Sam.
The New Testament has a similar warning about the danger of believers being bound to unbelievers. Though not explicitly about marriage, 2 Corinthians 6:14-16 lists a series of questions: what do righteousness and sin, light and dark, Christ and Belial, believers and unbelievers, or God's temple and idols have in common? The obvious answer is nothing. The rest of the text is about the need for purity and separation from sin (6:17-7:1).
If you've seen the movie you remember Sam's broken heart as he turned back; rejected, separated from his friend. Imagine then how Jesus feels when we let the Gollums of this world persuade us to send Him away. His heart is broken because He wants us for ourselves (Hebrews 2:9-18).