Wow - all this stuff blows me away. To the OP-er, you are to be commended for having a calm, level attitude about this.
This is my experience with my stepdaughter when she lived with us briefly. She was 18, going out with her first boyfriend, who was going into the Air Force. His parents were apparently permissive, because she lost her virginity to him, in his house, while his parents were there. I found out by asking her, because I had an instinctive feeling that something had happened. She claimed in the beginning that they had used a condom. Since she seemed to be serious about this young man, I took her to get birth control, and with the doctor's help, explained everything she needed to know. I stressed the proper use of condoms. Her own mother had never spoken to her once about sexuality, consequences, how to protect oneself, etc. Since then, she has left, broken up and shacked up with another loser boyfriend, and is now back at her mother's house. At 21, she is still very immature and irresponsible, doesn't work, and doesn't really have a clue what to do with herself. We also found out that she did not, and still does not use condoms with these young men she consorts with. She said the condoms were "icky" and uncomfortable. The birth control pills she was on caused her mini strokes, and she lost partial vision in one eye, so now she is not allowed to drive.
Your daughter is quite young. Fifteen is a very young age to be sexually active. Where is the boyfriend in all this? I would focus on getting the young man involved with this responsibility too, along with his parents. If they are sexually active, do they do so at a friend's house, or his parent's house? If you disapprove of their sexual activity, I would tell both of them so, that they are too young to be involved this way. But if they make that choice without you, I would insist that the boyfriend be a man and pay for your daughter's birth control. I would also insist on 1) HIV and STD tests for both her and him, and 2) that both attend some kind of sexual education class, to make sure that he knows how to use a condom properly, and that he uses a condom each and every time, whether it's with intercourse or oral sex. Be up front and unrelentingly personal with him, if you can. Is the boyfriend ready to be a father in case she gets pregnant? Another important part of responsibility.
I know all this may sound too gross and personal, to think of one's teen engaged in this activities, but if they already have the birth control, then they are doing the unthinkable, most likely without care or skill. In this day and age of life threatening STD's, it's your daughter's life at stake.
Good luck with everything.