I personally don't have a problem with 17-18 year olds sleeping together in the same bed/having sex because most of them are going to at least have some understanding of the fact that having a child that young would slam the breaks on any future they might be planning.
Being southern myself OP, I'm a little bothered that you seem to think that our customs are really all that different from yours. It's not a regional thing, it's typically a religious thing. If her family is Baptist, and there are several kinds, you were lucky to be able to go there period. The same could probably be said about any strict religions, like Mormonism, Catholicism, and many other Christian ones, and I be willing to bet religions other than Christian as well.
I recently took a trip with my gf for a week to visit her family, and we slept in different houses. Mostly because her dad is just an overprotective asshole, but that's a story for another time. XD
Her mom wouldn't have cared if we slept together, as her mom loves me and thinks I'm probably the best thing that's happened to her daughter. Her mom gave us as much free time together as she possibly could, whereas her dad seemed to be calling every five minutes, which makes it very hard to do anything even remotely intimate with your significant other. I swear, it was like her had cameras in her mom's place. I kissed her, and my phone rang. We started fooling around, my phone rang. I was tempted to turn it off, but I didn't want to wake up dead. XD