Don't worry about being the strict parent. She may resent you at first, but eventually she'll see that you have her best interests at heart. For now, tell her that you want her to have the longest and happiest teen age years she can possibly have, and that the best way to do that is to not move too fast.
I would not be happy about a 13 1/2 year old having just one boyfriend, although they are pretty hard to reason with at that age. If she is starting to "pair off," it's probably time to sit her down and tell her that you hope she will wait a few years before having sex, but that if she does have sex, she MUST protect herself with condoms, and the pill, and that if she gets into trouble, she must NEVER be afraid to come to you for help.
One of the things I did with my kids is to start conversations about kids we'd see, or kids we knew, who were having sex too early, and/or getting pregnant--- I would say something like, "My god, I hope that little girl isn't the mother of that baby she has in the stroller! She's really going to miss out on her teenage years!" Or make comments about friends of theirs who are so caught up in their exclusive relationships that they are missing out on good times with their larger group of friends. I made a big example of a cousin of theirs who had a baby and just wanted to leave it with her mother all the time because she missed going out and partying with her friends.
I was happy to let my kids go out in groups of kids at that age, and do things like bowling on Saturday, so long as it was in a group of kids. It's not a guarantee that they're not having sex, but at least they're not all-absorbed with each other, to the point that no one else has any influence with them. It's hard--- good luck.