ETA:
From what you're describing, I think the day trip to mom's place sounds super fun, and would have done what you did - go with him for the day. I just think sending overnight is out of my comfort zone. It's giving permission for something I got when I was older, and quite frankly, I wish I hadn't even got it when I got it, looking back. So I am basing it off my own personal experience. My mom was too lenient. I think boundaries are kind of good.
Added:
I don't know about everyone else, but I find the whole concept of kids having girlfriends at age 15 these days and how serious they are (at least around here) kind of unsettling. My son's best friend has a girlfriend like this, and it's like they will get married some day. They were having sex (definitely) at age 14. They would do this sort of thing, and while both kids are nice kids, and I like the parents, I just think "whoa .. slow down" and I wish the boy was more available to hang out with the group of guys.
What Suz describes - piles of kids - that's one thing. I think pairing off - as girlfriend/boyfriend on couple trips with families at age 15, is just .. I don't know. Different. I see it happening with my son's friends, and I don't get it. He has two good friends like this, and those boys seem to be missing out. I know the guys miss their buds. I know the moms kind of question it and think "Well, why not.." but it's bit by bit .. and then they sort of feel peer pressured, and their kids bug them, so that inch by inch they seem to give in to it.
I am the youngest in a big family, and my mom, by the end of all the kids, gave in to me. So she was too lenient and I had a boyfriend very young. It wasn't as young as 15, but for that time it was young. I regret it. I wish she's been as tough/strict as she was with my eldest sister. Oh well - you can't change it, but I wish I'd hung out with my gal pals more back in the day. You get one childhood.
That's just my perspective. I think kids are too romantically involved these days with just one kid. Groups of kids hanging out - not seriously in couples, is one thing. My kids have friends of opposite sex. Girlfriends and boyfriends spending weekends together at age 15, to me .. is just different. I get it's the fun outdoors, and what's the harm .. it's just .. something funny about it, to me. Just my opinion :)