The third one (spoilers)
This was definitely the most interesting one, which seems to be the consensus here! Clearly the ending is left up to interpretation, but I was wondering about another part...
When that other guy was over and they went off to have sex without the boyfriend, he went into his doghouse room and sulked for a bit, but then put on his doggy mask and crawled into the bedroom. My first thought (and I think there was eerie music playing that alluded to this) was that he was going to attack them (as a dog). But then he saw that they were already finishing up and he just looked sad and took off his mask.
So what do you think? Was he going to attack but the sight of them enjoying themselves/climaxing threw him off his game and he changed his mind?
Did the sight of them having sex give him an even BETTER idea?
Or had he actually decided to just join them as a doggy and then was sad/vengeful that they were already finished? (Thus causing him to realize he wasn't needed and everything was always about her)
And one more thing I noticed....that dog mask looked like it came from an actual dog face! When they showed it up close for a longer length of time, you could really see how uneven it looked, like it had actually been made of animal skin. I wonder what the backstory on that was!