Both me and my best friend (and a few others in our close friendship group) "lost it" at 14.
My best friend's mum was of the "they're going to do what they're going to do, and I'd rather they had all the facts, a safe environment, and an experienced ear to talk to" school of thought. Best friend was on the pill, always had condoms, boyfriend was well known to her etc.
My mum was... well she was intense. She'd had a mum exactly like my best friend's and went polar opposite on me - as parents often do.
When my mum inevitably found out I'd "done the deed" (because mums always find out) she was extremely angry, embarrassed me, contacted the boy's parents which embarrassed me further, grounded me for a month, removed my TV, and even when I finally escaped the grounding I had severe restrictions on basically every aspect of my social life (which I found devious ways to get around i.e I'm staying with X friend, I would call from X friend's house phone to confirm we were home, then spend the night drunk in a field).
I can't even explain how much I wished I could have had my friend's mum, and how much her parenting approach (not just to sex, to basically everything!) fractured our relationship.
As a parent now, I can absolutely see she was doing her very best to protect me and I respect her for that, we are extremely close now.
But as a teenager I really resented her. There were times when I really needed her (i.e, after I was raped) and I felt like I couldn't talk to her because I wasn't where I said I would be, and thought I'd be in serious trouble. I moved out and in with an older friend (who basically had a party house where drugs were rife) the minute I turned 16. I came to my senses eventually and moved in with my grandma, and things got a lot better.
That was pretty long, and extreme. But my daughter is approaching that age soon (I was pregnant at 17... yup) and while I do not want to be her best friend, every parenting decision I make regarding how much freedom she has, and my reaction when she inevitably fucks up, is based on the knowledge that if she wants to do it, she will probably find a way, and if she does, how dangerous is that going to be, and is my behaviour going to influence our relationship going forward?
Do I want her to be out drunk in a field, or do I want her first experiences of alcohol to be at home, with me, where she is safe and I can limit the risks?
Do I want her to lose her virginity at 14 in the back of an allyway, get pregnant, and not be able to tell me about any of it? Or do I want to accept that 14 year olds have sex, help her with contraception, educate her on consent, and look the other way if she's in an environment where she feels safe and also confident enough to say no. If a teen feels like things are going too far, or changes their mind, are they going to feel more confident saying that when it's just the two of them in a house, or out in a field with a big group of peers all getting up to the same stuff in the near vicinity?
Basically OP I would tread very carefully and perhaps consider reframing from "how do I stop them having sex" to "how can I protect them best?" And there has to be an element of assuming that at 14, they will find a way to do it anyway if they really want to.