[Oblivious]

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Turning it to what they were doing or weren't doing is disaster in this situation.

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Regardless of what was actually going on, the OP didnt do anything wrong. It would have been bad if he failed to apologize for walking in on them, but he of course did apologize.

The fact that the roommate and his gf are making such an effort to come up with an alibi is a big tell, and explains why she reacted the way she did. She obviously feels like a dirty whore for getting [censored] on the bathroom floor.

She uses two tactics in some attempt to prove shes not a deviant.
1)Makes it your fault for walking in on them. To hell with behaving like a guest. She has to get really pissed at you so you'll be convinced you did something horrible, and that she's innocent.
2)Denies that she was getting it on at all. Sew the seed of doubt that she was doing anything distasteful.

If she doesnt question her own virtue, she'd never lie in step two, since theres nothing wrong with [censored] your bf in the bathroom.

Girls have a tendancy to feel guilty and think of themselves as dirty sluts. They want as few people as possible to know about their sluttyness. This whole problem is not about you walking through an unlocked door, its about this girl's self-esteem.

Maybe sit this girl down and tell her, "Its okay! I have sex with myself all the time in there."