I’ve just “hung out” with friends of the same and opposite sex well past midnight plenty of times. Nothing bad came out of it. Teens need to socialize and during the school week it’s hard, so we like to make up for lost time on the weekends by staying out late.
Same and I would have found it devastating if someone had told me he didn’t believe my word on just being friends and acting decent with a lady friend. Then again, those who think they stand may they watch lest they fall.
If the friend’s parents are in the house, that’s actually safer than dating, where there’s no supervision plus there’s a lot of pressure and awkwardness, which prompts young, especially unformed minds, to not know how to act and thus end up doing stupid things.
Besides, I think teenagers have business having friends, while they mostly don’t really have business dating. It’s not like they’re going to count until 3 dates, then start a relationship, then get engaged after like 6 months and married after another 6-12. Young people really, really need to get to interact with the opposite sex without such pressure.
Even right now, as a man closing on 30, I still prefer “just hanging out” to artificial scenarios that don’t tell me much about the other person, and I could never make up my mind to come up with any sort of credible conclusion in such circumstances. I don’t think I’ve ever used the word “date” with a woman if not in jest.
What I like about normal, unforced contact is the shared histories that form between people. Friends end up having histories together and so do those who actually do marry each other in the end… as opposed to coin flip decisions that people end up regretting (because they marry without knowing each other, only having gone through a short checklist while dating).
But obviously, it’d be a good idea to know what opposite sex teenagers are doing, without inappropriately invading their privacy. Especially when hormones are raging, the local secular culture is permissive, the individuals have had previous incidents of poor judgements.
Same boat here. Our two oldest are our 17 year old daughter and 14.5 year old son. While both of them have been out in mixed company neither of them are allowed to “date”. The one time it came up with our daughter I asked her point blank if she thought the boy was someone she might want to marry. That definitely got her thinking about the purpose of an exclusive relationship.
I’m not American, so maybe this is just a cultural difference but I’ve never been able to figure out what’s that with the “exclusive” vs “nonexclusive” distinction. As in, if it’s “boys and girls” things, like kissing, reading love poetry, generally behaving like loving doves, then it obviously should be only with one person at a time (as in it doesn’t become better when the same starts to be done with 1 or 2 aditional people apart from the first or the main interest… which would get my hypothetical child into a very serious conversation), while on the other hand if it’s just friendly banter and maybe just a little dash of flirting, then the entire question of exclusivity is pretty much moot to me as there’s really little to be exclusive about.
On the other hand I understand that it can be detrimental to the development of young people if they act like they’re married or engaged and overinvest emotionally, close themselves to meaningful opportunities etc.
In a few years, they can do whatever they want to do and you’ll have no control over it, so make sure you don’t give up your role of parent so soon! 
If they are always together, which GF’s house are they hanging out in? Until midnight? Invite the girls over to your house for the evening. Invite their parents too. Have a party, with adults and kids. Get to know the parents of these girls so you can find out if they are being supervised or if it’s “anything goes.”
I’d even hang out at his house sometimes too, and the same occured. Mostly then it was me and his 3 brothers hanging out together.
