Also she wants to go on birth control so she doesn't get pregnant. Even though its not 100% effective. I want to give her a moral reason that she is set apart, our bodies are to honor God but I cant compete with the message of the world
Moral reasons, like being holy or honoring God alone rarely work very well when it comes to sexual matters... as
1. Human desire runs very high, especially for younger folks.
2. The world says emotional investment in a relationship brings about lots of hurt when it fails, so you better explore sexual compatibility before you get too emotionally invested. Thus the 3 date rule...
3. Christian divorce rates are little different than the world. Sexual assault and adultery in the church exists all over the world, in every state, and likely in every county. STD rates in the US Bible belt have reached levels never seen before. This sort of stuff kills the churches moral teaching authority. Sure, the scriptures may say one thing, but when the church acts counter to it, what are kids to believe... and maybe your church is still free of scandal, but this doesn't mean your daughter hasn't heard of a church that she couldn't drive to in under a day that has.
I once worked with a young university student who was working on mission development with her boyfriend. An amazing couple from an uber-conservative church, they'd often go preach on abstinence to other students and were very well regarded... until of course she got pregnant. It took a lot for me not to blow a gasket... but what can you do but love and support them at that point.
The last stats I read said something to the tune of 92% of born again Christians engaged in premarital sex by the time they reached 25. The figure seems a bit high based upon my personal experiences working with young adults, but whether its 92% or 70%, the church is failing in this.
Which leads to another problem, rather than relying on God's word, many well intentioned folks start telling lies, inadvertently repeat the lies of others to encourage waiting, or twist the scriptures turning virginity into an idol. This sort of garbage can prevent premarital sex some of the time, but it can also serve to doom marriages before they even get started.
I think a better approach is the use of reason and logic in combination with the scriptures.
1. She may want to have a good marriage at some point, with little to no fear that her husband will cheat on her, or that she would ever be tempted to cheat on him.
2. Behaviors do not change when folks say I do... if anything, they revert to a lower common denominator, as folks get more relaxed and are less willing to put on a show. 3. If you don't have a healthy sexual ethic going into marriage, saying I do won't change a problematic one into a healthy one. This is true whether folks have engaged in premarital sex or not. It needs to be developed over time, which will require self control.
4. Self control in marriage will be needed in times of rough spots, especially so when temptation rears its ugly head at the worst possible time... leaving one in peril of possibly not fleeing from temptation.
5. By not engaging in premarital sex, she is practicing self control, and the skill she learns in doing so can provide a lifetime of benefits whether she marries someone or not.
Tell her to ask here peers who have had a relationship last beyond the romance hormone stage... ~3 years as to whether sex is the glue that keeps it together. She will find it is not, and likely she will hear stories from believers and non-believers that building a relationship on sex alone is a really really bad idea as it's not sustainable.
So, if she loves someone, is she going to jeopardize this by sleeping with them early in the game which is likely preclude putting in the relationship work necessary for it to survive? If she really loves a guy, is she willing to put up with the heartache and potentially ruined plans of her and her boyfriend should their birth control fail?
Since she has drifted from God, appeals to personal holiness, and/or honoring God are unlikely to work, and should she still engage in premarital sex, can serve to put up barriers in her relationship with Him. So yes, by all means explain this... but don't pound it into the ground.
Its really awesome that you two are communicating with each other about this. Far too many parents and kids don't often to mutual peril. As far as if she does.. love her, just as you do now. If you find the ongoing practice personally offensive, you will need to decide if its ok if she and her boyfriend engage at your family home, or whether to provide them separate rooms, or ask them to stay at a hotel somewhere. Pray for her, pray for discernment for yourself as to how best to love and support her no matter which way she decides.