You needed a roommate and Sophie was recommended to you by a female friend at your job. Sophie seems nice, but you and her had to establish a few ground rules. The main thing was that Sophie felt your relationship should be platonic so that there wouldn't be any confusion regarding the bills.
You are OK with that and over the last few months you've entertained a few ladies at your place while Sophie was home. She never complained, but you noticed that Sophie is starting to dress differently around the house lately.
Keep in mind she set the ground rules.