There is a proverb: "Food not eaten on the table, will be eaten under the table as crumbs."
What I'm reading, is the break down of communication. Everything else is merely the symptom of a communication break down. I'm sure you don't need anybody to tell you that. Your text seems to indicate it, when you said you might need to start talking about subjects like BGR.
You recognise that there is a need for transparency, so that you can make the right judgements for your child. Transparency needs trust too. I'm going to sound odd, but as parents, we need to earn our children's trust too. They are not our robots who do our bidding, but someone who is going to be flying out in the wild soon. I get many hints in your text that, you place expectations for your daughter to do your's and your husband's bidding.
BGR is not something that your daughter can avoid, neither should she avoid. After all, it's how you and your husband got together in the first place. Don't put up red tape everywhere.
Teach her how to judge a boy.
Teach her how to love and receive love.
Teach her to say no.
Teach her to tell the boy, "my parents already validate me, I don't need yours"
Teach her that she always belong to the family, not in the arms of another boy.
Teach her that there is always love at home.
Teach her to expect the boy to respect her boundaries.
In fact, I'm going to suggest something against asian culture. Bring that boy home for dinner. As a parent, get to know that boy yourself. You're a mum, come on, you know what's a good boy and what's not. Your daughter might not know. This is your time to show your daughter your wisdom. Show with her your judgement and the rationale behind the judgement. Teach her to tell him herself the boundaries of relationships in your presence. Teach her to set the terms and conditions of this BGR.
This is how you show your daughter (and your other 2) maturity, confidence, and independence. Your daughter is going to use that exact same stuff you teach today for her future marriage.
Eventually, you are going to need to touch on other sensitive subjects, such a pornography, sex, masturbation, LGBT, etc... It can't be avoided.
Can your daughter (and your other 2) trust you enough that she be transparent to you on the table, to seek you for guidance as she goes through these subjects? Or will she shy away from you, do it secretly under the table, learn from friends and internet, and potentially hurt herself?
You are already a wonderful mum, protecting your kids from many dangers since young. Your daughter is coming to an age where she needs mental maturity, and you are the best person to learn from. It's time to adjust parental style. Give her the phone and ask her to invite that boy for family dinner.
May God guide you as you parent your precious 3.
Grace Chai