Being in love with your clone was wrong, end of discussion.
It's not like Sarah was in love with her, because she wasn't and that's a terrible accusation. But one night Sarah woke up horrified after having a night of passion with Alison in her dreams and thought she was going crazy.
Alison, of all people. Sarah could have understood it if she had dreamed about Cosima, or Paul, or God, even Rachel.
She couldn't fathom why her brain chose Alison.
(Until she finally did, and how many times did Sarah have to say this? She was absolutely not in love with Alison Hendrix.)