I tried to explain it below, Raptus had the wrong angle I think. Its not that there wasn't time to explain, its just that the fear of the reaction was overwhelming.
I didn't quote your first post, but you are off. Its not that she didn't have a chance, but rather she was legit terrified. I didn't wanna get into this debate, but I will say this. Its not that she lied, its WHY <--- important. Literally her real family turned their back on her (minus Derek), and given how both girls act, their friends did too. They got absolutely shunned and their whole world got turned upside down. I'm just saying, she had plenty of motivation to keep quiet, especially with the line to her brother "This is just easier", she was accepting that maybe being with a guy really is easier, shes THAT scarred from her experience.