The information given out so far doesn't mention sex. Info is that the victim and killer were roommates in a dorm room, the victim reported a theft by the killer, the killer forged a suicide note purportedly by the victim that stated the victim lied about accusing the killer of theft, and the killer repeatedly stabbed the victim while she lay sleeping in their dorm room
“”What is your basis for ‘sex’ as a motive?””
If you read the entire thread you will see that link in post 3, the girls personal web sites give clues to what happened, which gives a base for speculation.