The sex scene was ridiculous
There is no way any reasonable person would have sex at that time. The trauma alone would sufficiently prevent the very idea; at this point, she had drunk the smoothie, saw the acid scene, saw the tooth pulling, and killed her dog. It's too much for anyone to look past.
Also consider that she knew the killer has cameras, so she knew she would be having sex in front of a psycho. It was also the 4th day of captivity, and she had not showered, brushed her teeth, or otherwise maintained herself respectfully enough to get naked in front of another person. Her breath must have been something after ingesting human remains!