Is dating unbeliever actually sinful ?
( serious question)
If a person will not trust in Jesus, what would make that person desirable to you for dating?
An unbeliever is a child of Satan. So, why are you attracted to a child of Satan, instead of to a child of God?
Why would you not date a person who is helping you to grow in Jesus and who can be a good example to help you bring up your children?
I have known a person who wanted to have someone when she was old and retired. She picked up one guy in a bar. It did not work out. Then she got with an old friend who was not saved. I offered the unequal yoke thing, and she talked her way right around that.
After their pronouncement things got troubled quickly. I was not surprised.
He was not her real problem. She needed to get wise to what made her able to get herself into wrong relationships.
Already, I could see she was a controller who could make things happen the way she dictated. Except, in a close relationship, that charm and cleverness didn't work so well. And she could justify other things, like bar socializing. So, she was her real problem, then.
So, just having a rule, "don't date unbelievers", could be legalistic, unless they are providing ministry for how the Bible says for us to relate as God's family, and how to make sure with God about whom we date and marry.
She clearly did not make sure with God. And she was not a role model for Ephesians 5:21 and James 1:19-20 and Philippians 2:13-16.
Christian dating and marriage include mutual and prayerful submission (Ephesians 5:21). And we lead by example > 1 Peter 5:3 > not using each other and pressuring and charming each other to do what we want.
An unbeliever is incapable of genuine mutual and prayerful sharing. So, they are incapable of God's way of love!!
So, if you can ask if it's "sinful" to date a child of Satan who does not know how to love . . . possibly this means you need to spend much more time in prayer and God's word and in sharing with mature Christian people who do not only have rules, but they are examples of how to do well in marriage and other close family relating in Jesus > including >
"Let all bitterness, wrath, anger, clamor, and evil speaking be put away from you, with all malice. And be kind to one another, tenderhearted, forgiving one another, even as God in Christ forgave you." (Ephesians 4:31-22)
I mean older people with decades of maturing.