In the end, it seems a lot to be about STDs and men are somewhat feeble creatures. They will knowingly hide and spread them if they can? They will also take part in risky behavior with the knowledge that they may be in danger from their partner? Are women more honest about it in that they will communicate the risk before engaging with somebody?
Overall, I really enjoyed this without thinking about all what it may be trying to say, it was quite tense and creepy for most of it but started to run out of steam towards the end. But it still had a better them most ending for a horror film. While the thoughts of STDs started to come to my mind part way through, I didn't try to think of it that much until afterwards.
My wife loves horror films, so will save the second viewing for her