[jet]

Steph was the one who cheated on her boyfriend, so she showed no concern for the man she has been with for a year, living with day after day. Jeremy had no emotional connection to her boyfriend who was just an anonymous person he could brush off much easier. Jeremy is not much better than she is, but she is the one who had the memories of her boyfriend, and the times they spent together to stop her doing the dirty on him. Her loyalty lay with er b/f at least until the show was over and she could end it in a dignified way.
Apart from the boyfriend cheating, she has shown herself to be lazy (have you ever seen her doing any cooking or cleaning?) she is unhygienic (knickergate and razorgate) she is self centred, she is selfish and she behaves like a child whenever she doesn't get her own way. She can be annoying and provoking and attention seeking.
Jeremy came across as more mature and a steadying influence on her, so of course he comes out of it looking better.