The goal here is not to undermine the parents values and encourage the teen to sleep around. The fact is that while some teens are not sexually active, many are, either with or without the parent's knowledge. Sexual health and history is a very important part of your exam, and for you to help the teen achieve his/her optimal health, you need accurate and honest information about their sexual history. This is something they may feel the need to lie about in front of their parents. This is the ONLY reason you are asking the parents to leave. To facilitate honest communication. If you feel that it is important that the teen discuss this issue with the parents, you are free to encourage the teen to do so, but remember, he/she is your patient, and confidentiality applies. As far as contraception is concerned, laws on the matter probably vary from state to state, but either way your obligation is to advocate for the patient's health, and if that means providing a discreet contraceptive script, then that is your duty. It doesn't mean you urge the kid to go out and screw, in fact you can and should counsel them on the dangers they may be getting into. But you have to accept the position that the patient is in, and do what you can to help keep him/her safe and in good health.
Sadly, many women ARE in abusive relationships. This can be uncomfortable to acknowledge and certainly is a tough thing to discuss. But the fact is, many women are abused and many women will not necessarily volunteer this information. Again, remember that it is your job to advocate for and fight for her safety and health. This means that if you have any suspicion AT ALL that somethign like this may be going on, it's your responsibility to delve into it. This doesn't mean you're 'accusing' someone. It just means you're checking off all the possibilities. While it may be uncomfortable, the first time you find out that someone IS being beaten and can help that woman, you'll understand why it's worth it.
A complete medical history DOES include a sexual history. Do you think that just because that 75 year old hasn't had sex in the last week, that means that nothing in her sexual history could possibly have an impact on her current health? More importantly, do you really think that a 75 year old widow can't possibly be currently having sex? Plenty do..
And good for them! Being a widow doesn't mean SHE'S dead, after all, who's to say she hasn't met someone new! Also, there are plenty of people who are homosexual. This has implications on possible disease. There are plenty of bisexuals. ditto. Lots of people have been raped, both by the opposite sex and the same sex. This is important to know. These things are important to know so that you can insure that their sex lives are healthy and fulfilling. they're important to know so you can be aware of the potential for stds and other sexually related disorders. They're even important to know because sexual dysfunction can be a sign of other, non sexually related, disorders such as heart disease.
In the future, when you're being told things that you feel are meant to brainwash you into being a pinko, why don't you ask the lecturer for clarification, either in class or privately. I think you'll find that there really are good reasons for most of what they are telling you.