tl;dr ahead -
I think the best sign the daughter is not ready for a mature relationship is the fact that she disobeyed a rule, as someone else brought up. And as even sven said, you’re her parents, and you seem to have a close relationship (which is really great), so you can be a judge of this.
As for the emotional risks, regardless of whether or not this is an absolute truth, our culture places a high value on the intimacy of a sexual relationship. Allowing someone else to be intimate with you in that way requires a great deal of trust. You’re trusting the other person to be sensitive to your feelings and what you’re ready for, and to continue to be loving and supportive of you even after this dynamic has changed in a relationship. Having this trust broken is very painful and for someone young, can be bewildering and overwhelming. Entering into a relationship always means allowing yourself to become vulnerable to some extent; the more intimate, the more vulnerable. That’s not inherently a bad thing - it just means you have to be careful about how you approach a serious relationship. I definitely didn’t know that at 15, and I probably didn’t know how to choose people I was willing to reveal that side of me to.
The flipside of that is that at 15, especially with our rather broken educational system (I’m assuming you’re American), a young person may not fully understand what consent is and how to be sure that her partner is actively consenting - how to check in and make sure her partner is okay at various intervals - and how to make sure that she isn’t pressuring her partner.
I think her age is not what makes her not ready. What makes her not ready is that, as someone who has not had a lot of interest in a sexual relationship before, she doesn’t really know about this stuff! I’m Not Dennis Bergkamp, I think you and your wife should continue to have an open dialogue with your daughter on these issues as well as the physical ones, and as some other people in this thread recommended, she should start seeing a gynecologist.
Because it is true that she has now started a sexual relationship, so she still needs to know these things, regardless of whether you approve.
I agree with the people who have said she isn’t ready now but she could be ready after taking such steps. (I disagree with Lasciel’s suggestion that she do it all by herself. I think having parents who want to be involved in making sure she is doing things responsibly makes her really lucky, so don’t squander that.)
There is also a great value to being parents who set rules, and enforce them. If your rules get broken, and consequences are enforced, your daughter won’t assume that she can get away with whatever she wants. Even having some rules that she may deem arbitrary can be good for her. It is kiiiind of a thing that kids do naturally push boundaries, and keeping the boundaries a little closer than is strictly necessary means that she can do that without causing herself a whole lot of harm.
My parents were very strict with me until I left for college, and when they did let up on rules it was based on how old I was as well as how mature they observed me to be. I still disagree with some of their choices and if I ever have children I won’t enforce all of their rules. But I’m old enough to realize now that they were so strict because they love me and wanted me to enjoy the best possible quality of life. Someday your daughter will see that too.