9:00 EST is primetime adult viewing time, and CW is a cable channel, not a major network, so the same rules don't apply. But even so, nothing forbids a couple from having sex any time on US TV, as long as just suggested and not total nudity. Network soap operas are steamy and they're in the middle of the day! I think the writers said they did the cartoon because they wanted to leave it to the imagination.
CW is mostly a broadcast network, but they do have cable channels in some of the major markets where there weren't enough broadcast channels for them to obtain a broadcast affiliate.