I think the emotional risks of sex are the same at 15 and 50. Age really has little to do with it. Experience and emotional stability/maturity, does. Some 18-year olds may be more emotionally stable/mature than some 15-year-olds, but all older people are not better equipped to manage sexual relationships in a healthy manner than all younger people. Plenty of people are never very emotionally stable or mature, and struggle with sex and relationships their whole lives. I’d put a majority of people 25+ I know, into this category. Apprently they learned little from high school because they still act like stereotypical 15-year-olds when it comes to who they are dating. Lots of drama ensues.
Experience in handling relationships/sex does make a big difference, IME. So prohibiting kids from beginning to get this experience when they have the desire and feel ready works against the ultimate goal. Again I think it would be weird for the OP to enable his daughter in this particular situation, and I think ‘I feel uncomfortable giving you two permission to do it while I’m upstairs’ is the only explanation his daughter needs.
I don’t feel my decision-making or emotional maturity has changed much since my teens, though. I’ve always been very cautious, and I didn’t have much adult supervision of my free time starting at age 4-5. I had my own social life during my childhood and teens that my parents often knew little about, and I managed it to my satisfaction and have very few regrets. So I’m sure that influences my opinion here hugely. Other people feel they were different people as teens, with no understanding of the consequences of their actions, who took insane risks and needed close control from their parents. YMMV
Also, everyone lies about sex - to their parents.