I'm surprised by these responses. She's 18, not 14.

I wouldn't mention anything about the incident. I would instead talk with her about this boyfriend in a casual, getting to know conversation. Ask her what he's like, where's he headed after high school, if they're planning on still being together after high school, etc. Then ask straight out if they're sexually active. Briefly talk about the importance of birth control.. and hopefully you've had all along. I'd then extend an invite to have him over for a bar-B-Q.. perhaps with a few other close friends of hers.

I think I'd rather my daughter have sexual activity at home (discretely) than at some unknown location. I'd want her to be prepared and safe. It's one thing being caught by mom, it's another being caught by an officer for doing so in a park.