I think if their talk is loud, bordering on shouting, then I don’t think you are being unreasonable. If it’s just ordinary volume, then I think yes you are, a bit. The problem is that on still nights, voices carry further and can seem louder than they perhaps are. Actually, 12.15 is not unduly late for a Saturday evening.
If there is loud music, too, then I can understand why you might be irritated, but I’m assuming not, as you didn’t mention music. As others suggest, maybe close the window and use a fan or leave window ajar but wear some earplugs?