Honestly she doesn't have much ground to stand on. Having sex between two adults that both agree with the act in a private manner isn't illegal. Having sex in your house or in front of the window isn't illegal. And the comment about your son being there? Parents have sex while kids are at home all the time. The thing is, how big is the chance of the kid seeing it and being shocked or weirded out by it? Like you said your son was asleep, and locking him in the room would be not only unnecessary but also dangerous. Sex is a natural thing to be practiced inside. As far as I see it, you didn't do anything illegal or endangering for you, your son or girlfriend.