I guess that is what I would most emphasize: share with her your genuine concern that she make choices that support her and tell her that to that end you will set certain rules with certain consequences, but then acknowledge her responsibility for being the one who ultimately will make these decisions. PS - Alanon might be supportive for you as the parent, even if she is not using any substances, the issues are close enough.
A lot of oral sex goes on in dating, with definite health risks, loss of reputation, etc.
She needs to prove to you and herself that what you think, is of no concern to her - but it is a ruse - it matters more than she can accept or let on.
If you are not condemning (and sounds like you are not) but instead acknowledge the fact that this is ultimately a choice she must make about her own body, she will be more able to hear the things that you want to say about the situation.
Past efforts to monitor a dating situation have failed, either because of being lied to or because of being let down by a past boyfriend's dad who promised to supervise if the two spent time at his house when he WAS there and did not. Knowing I cannot control what she does, I want to set age- appropriate parameters and attempt to hold her to them. Also, these parameters cannot be based on expecting truthfulness or abstinence.
They have to be based on something else, giving her reasonable guidelines within which to learn to take responsibility for herself.