Update: The folks at BugGuide.net came through again! This is a Marbled Orbweaver (Araneus marmoreus), a spider whose abdomen may come in many different marbled colors, but which always has a reddish carapace and striped legs. Since it captures prey in a web rather than by stalking, perhaps spider and beetle were together only by happenstance. Although I suppose the spider might take advantage of the situation. Or do Japanese Beetles taste bad?
Is this a typical mating position for Small Milkweed Bugs? To find out, I googled "milkweed bugs mating" and discovered that yes, this is indeed how they do it, and also that they can stay attached for as long as half an hour. The male Milkweed Bug is smaller than the female.