at the risk of a hijack, can you elaborate briefly on this statement?
I think a lot of this is just build-up and self-fulfilling prophecy. Sex is just one of the things that people do on a daily basis, and it should be treated as that. No question she should be properly armed with knowledge about birth control and disease prevention, but the act of sex itself shouldn’t be given this aura of walls tumbling down. Adults fuck. A 17-year-old is entering the world of adults. She should fuck, if she wants to.
It may be good to bring up the HPV vaccine also.
Good advice and thoughtful opinions have been given, so I’ll just comment that your daughter may be more interested in regulating her menstrual cycle via oral contraceptives than in pregnancy prevention. If she were my daughter, I would make sure her health care provider made it very clear to her that oral contraceptives do not protect against STDs. Further, while STDs such as chlamydia, gonorrhea, syphilis and HIV are serious, she also needs to be aware of HPV, which has been linked to cervical and penile cancer.
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OK, OK, I’m done.
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There is a difference between creating space for them and condoning something. You don’t mention it at all to her, or him. No wink wink, nudge nudge. You just tell her that she’s fairly grown now, you trust her and will be taking your wife out occasionally for date nights. If the boyfriend is over, so be it. The point is not condoning anything, it is placing your trust in your daughter to make the right decisions. You don’t need to control this aspect of her life, and can’t anyway. All you are doing is giving to young people some privacy occasionally at your convenience. No need to schedule nights out, or be creepy or embarrassing.
Moved MPSIMS --> IMHO.
THIS.
Just go out, and tell her when you’ll be home (and don’t come home early!!). If you’ve never left her home like that before (why??) then say something like, “You’re older and I think it’s fine for us to go out while you are here.” Don’t mention the boyfriend, don’t mention sex.
You should be doing this anyway; forget about your daughter’s sex life, the date nights will be great for yours!
Here is a very recent thread which is very similar to this one. The difference being that OP’s daughter is 15. Is it wrong for 15-year olds to be sexually active? - In My Humble Opinion - Straight Dope Message Board
Some interesting & very wise points are raised there. It’ll pay our OP to read it.
Read this:
I grew up in The Netherlands and had sex for the first time in my parents home as a teenager. I cannot tell you how happy I am to have had that safe, happy, caring experience.
Your daughter sounds really amazing, you must be so proud that she is so responsible & that she comes to talk to you and your wife. I think it’s mainly that you need to get over the problems you have with this.
This is difficult for you, no reason to make it difficult for her. Good luck 
I would like to hear whether or not this was the case, also.
I concur. People make those jokes around me all the time (mine are 4 and 0, so it’s definitely joke territory), but I have every ambition of raising thoughtful girls with good heads on their shoulders (as it seems the OP successfully has). If they want to have sex when they’re 17, safely, more power to 'em, just the same as if they want to go on waterslides or play trombone. Sex is fun. But since talking to your parents about sex isn’t, my girls aren’t going to have to ask to go on the Pill.
–Cliffy
The OP is condoning the possibility his daughter’s having sex sometime soon. He’s just not particularly happy about it, is uncomfortable with it, and thinks it would be better for her to wait, but contra the OP, that doesn’t mean he doesn’t condone it.
I think you sound like a great parent, and it’s very much to your credit that your daughter was comfortable approaching you.
It’s completely natural to have some conflicted, difficult feelings. I’m the last one to bring up evolutionary biology, but there I think fathers are hard wired to be uncomfortable with seeing their daughters as sexual beings.
Of course, this is a stage she is pretty much ready for. I think you know this intellectually but are not ready to process it emotionally.
I agree with this. I’m no prude, but I would be extremely uncomfortable outright facilitating sex sessions for anyone in my family. I don’t think there is any need to play wingman to your kid.
But she is old enough to have some privacy now and then, and privacy means you don’t need to worry too much about what she is doing. I seem to recall my mother having the occasional night out and even weekend away, and she let me know I could have friends over as long as I wasn’t throwing crazy house parties. That’s all there needs to be to it.
I made those jokes when my daughter was about 2. When she got to dating age, I had come to realize that she would be having sex, and sex is a beautiful thing. I was never worried about her getting pressure from boys, as she got my “my way or the highway” attitude and a wicked streak of sarcasm a yard wide. If anything I felt a bit sorry for the boys that could not keep up. It was kind of fun watching her kick want to be boyfriends to curb, one after the other.
My advice? Have her get on the pill, and you and the wife can schedule a date night every now and again. Dinner and a double feature. 
ETA: Double feature means two movies, you sick bastards!
I have two daughters 13 years old. Not looking forward to the day I am posting on here like the OP. Good luck to you and try to understand that she is your daughter, but she is also almost an adult. You can’t stop this even if you wanted to, so be supportive in the birth control and talk to her about STD’s. You have done your job and hopefully she learned how to be responsible from you and your wife, time to let her fly on her own.
I think it’s pretty clearly spillover from the incest taboo instinct. Given how much time fathers and daughters spend together and the dynamics involved, there needs to be some pretty strong instinctual mojo happening that keeps fathers from wanting to have sex with their daughters. We are built to find this idea deeply disturbing on a fundamental level.
It’s not too big of a leap from “this young lady cannot be a sexual option for me” to “this young lady cannot be a sexual option for anyone.”
“sexual option”
I think I got that candy heart once.
Yeah, but mothers don’t freak out about their sons having sex and moms spend lots of time with their (male and female) kids.
Maybe because older men are more likely to be attracted to newly fertile females whereas women already with children aren’t subject to the same degree to spawn again with young males.