We are talking about FRUM boys and girls of an age to get married sitting at a table together, talking together, and eating a meal – NOT about going over to The Dark Side…
You assume that our young people are all just chomping at the bit to commit some unnameable aveiros. Maybe some of them ARE, and they would do it regardless. Most frum kids who are brought up in a normal environment where everything is not assered to them every five minutes (when it is not intrinsically assur from Hashem), do NOT behave in the way that you fear. Yes, maybe SOMEONE will, but it is not because of mixing the genders in a public place, it is because there is a fundamental behavior censor chip missing from them. JMO, and I grew up in a very frum environment where we ALWAYS had mixed groups. No one went off the derech, and I think we all made our parents proud.