"But we have so much in common ..."
"But he/she makes me happy ..."
Those are just a few of the simplistic excuses "Christians" give for marrying unbelievers (or for living with and being sexually active with unbelievers they aren't married to) in spite of the scriptural command to not be "unequally yoked" (2 Corinthians 6:14).
They don't think about the fact that real "love" doesn't lead someone to sin, or given a little time we discover we have something "in common" with just about anyone, and that being "happy" isn't something someone else can provide for us over a lifetime.
And what these excuses don't --- and cannot --- explain away is that massive difference that will always separate them from any unbeliever they would be unequally yoked to: the truth.
When we consider the words Jesus prayed for His disciples, "... so they may be one as we are one ..." (John 17:22), it is sometimes forgotten that in the very same prayer He asked that they may be made holy ("sanctified") through the truth (verse 17). The word used for "sanctify" signifies separation --- the idea is to be set apart for special use. So two ideas are seen to be in the mind of Christ as He prayed: first, that His disciples should be a separated people, and then united together in Him.
What the disciples were to be separated from is clear from the context, as follows:
"They do not belong to this world any more than I do. Make them holy [sanctify them] by your truth; teach them your word, which is truth. Just as you sent me into the world, I am sending them into the world," John 17:16-18.
Clearly, the disciples of Jesus were to be separated from the world, even though they were to be in it. This separation would result from the influence of the "truth" upon them.
"Make them holy by your truth; teach them your word, which is truth," says Jesus, so it was the power of the word of God upon their minds which caused the separation to come about. When a Christian insists on being unequally yoked to an unbeliever, they attempt to meld truth with a lie, the believing with the unbelieving. It's an impossible mix. There will always remain a separation, or even worse, the believer will surrender an obedient practice of faith in order to become yoked with an unbeliever.
Jesus prayed that the Father would make us holy by His truth, that God would teach us His word, which is truth. We either are made holy by living out the word of God, or sin by failing to obey the Word of God. To claim to be a child of God but then give yourself to one you know does not live out the truth of God is choosing to join with one in rejecting the truth from God.
Truth creates a gap between the authentic believer and the unbeliever that only conversion of the unbeliever can resolve.
And no, there is no such thing as evangelistic dating! That's just flirting with sin.