Ease Me In (The more guarded approach):
This is kind of a hard conversation to have with you, and believe me, I've gone over it again and again in my head. But I feel like it's important to be open with one another. I do understand your concerns with me having sex or using birth control, but I want you to trust that I am taking all the right steps to be safe and make sure it doesn't turn into something I can't handle.
Just Give It to Me (The no-nonsense tactic):
Being sexually active and using birth control has nothing to do with age, or status, or personal character. They are decisions that should be made on a case-by-case basis. In my case, I am enjoying my life and my choices. I hope that you can respect my decision, even if it is not what you'd choose for me.