After all, they are not in the Bible, and are not part of the Biblical model for finding a marriage partner.
I believe that they have no place in Christian life.
In my opinion, there should be no commitment until marriage is proposed and decided. The BF-GF relationship is just pointless, and is a false commitment. It wastes time if you're not actively seeking a marriage partner. God never meant for a couple to not know where their relationship was headed.
Therefore, I believe that dating should begin with the man stating his intentions (getting to know the woman and assessing their compatibility for the purpose of marriage), so there will be no guessing games, and there should be no exclusivity or conveyance of affection until engagement.
This is definitely not a popular view, so I welcome any comments!
Short answer: BF-GF and dating are allowed. To say they are sinful is being legalistic.
1) You don't even need to be BF-GF to sin. You just need to look at any random man or woman lustfully and you already committed adultery in your heart. (Matthew 5:28) And we all did it, many many many times.
I will say I sin this way almost everyday, especially in the summer when a lot of ladies wear their mini skirts. Even girls in church loves to wear them. I am not saying the ladies can't wear them. I am saying my own sinful nature is obvious.
Sin is not just the external actions. Sin include everything in our hearts. Sin comes from the heart.
2) Anything
not written directly in the bible as sinful is
allowed by God. There are some grey half-written rules, such as abortion. (Depending on what you consider a human life, which isn't 100% clear. And you have to consider the safety of the mother. The husband is commended to love his wife more than his child, relatively.)
However in the BF-GF case, it is certainly allowed. You have to understand that
according to the bible, arranged marriage was the only way. There was no courting. There was no dating. But that
doesn't mean God is against courting or dating. Back then nearly every culture does arranged marriages. India, China, etc. And they were nowhere near being Christians.
Joshua Harris himself apologized for writing that book on courting.
http://www.sgmsurvivors.com/2011/06/27/despite-apology-josh-harris-releasing-boy-meets-girl-again/
3) Sex before marriage is sinful.
This should be obvious. So let's say a Christian man dates a Christian lady. They had sex. Then they broke up a bit later. She meets another Christian man. And she have to tell her new BF that story.
How would that new BF feel?
If you are that first Christian BF, why would you
take something away from your fellow brother in Christ (her future Christian BF)?
Oh because you thought that she won't have a future BF? Why do you think that? What made you think you would never break up with her? You would be assuming too much.
Or maybe she is no longer a virgin when you start dating her. So that is ok now? You want to join that long list of her regrets? You want to just be another one black mark on her list?
Dating is allowed. But you must imagine that one day you might break up. And one day she will invite you for dinner with her and her new BF. What do you want him to say to you?
"Brother, thanks so much for keeping her pure for me! We are both so grateful!"
Or
"You are a sinner and false Christian! Thanks for nothing. You stole this precious thing from me!"