I'm not sure Maryann's "husband" is literally the devil but he's definitely in that league. Without giving away details, what's going on with her is far older than Christianity is and what/who she is trying to bring forth is definitely ancient evil/anarchy.
The series is good. It has a surprising amount of nudity and simulated sex acts but definitely not porno. Softcore at best in it's raunchiest and it's treated as a part of the series, not as an excuse to show people doing wild sex acts (Phlox's has to be the best/worst one yet) and being gratuitous.
It's on par with the amount of violence we see in the series, which itself is actually no more violent than some network series depict. The difference is in the graphic depiction itself. So for example, someone in a network series gets shot or stabbed, we may see some blood. In True Blood, it gushes more realistically or is more believably a wound of that nature.
I always find it odd people are okay with excessive violence but you show someone's tits or a sex act and suddenly it's taboo. Like beating someone to a literal bloody pulp is okay but seeing two people screwing is obscene and offensive.