The alleged sex occurred on new years in the bathroom, different night than the closet. They were both drunk and Trish had just finished the dancing challenge iirc. The evidence to support that this actually happened as I recall is:
- Brian has kept his story consistent and says that they did have sex on new years in the bathroom
- People apparently heard sex sounds coming out of the bathroom from Upstairs Hallway that night (Cannot confirm - never heard it)
- Trish answered questions about having sex with Brian during the lie detector bit "Was Brian good in bed?" for example - to which she said no. She could've been kidding but she did not say "that never happened", she answered the question.
- Trish freaked out when they gave her the fake positive pregnancy test so she was definitely afraid that it was real. This doesn't confirm she was thinking it was Brian's kid though. Trish could've easily thought she was knocked up by some guy from before the show but it was like 5 weeks into the show by that point.
So it's far from 100% confirmed, but i'd say the evidence is more on the side of it happening. She also just seems embarrassed when she denies it, like she's lying.