And yet, as I recollect, you are only forced to have sex with Jamie. I believe you can pick and choose with every single one of the others, even Shauna - she doesn't HAVE to become the MC's other girlfriend.
Jamie will always make the offer, regardless. It's up to the MC what he does with it though.
However, once you turn a girl down, you may not get another chance with her. So taking them on one at a time is probably out of the question for most of them.
Not a problem for me. I'm having the MC going after every girl that he can choose. I'll keep doing that until it bites me in the ass. Then I'll go back and change my selection and redo it. I have a save for every choice that even looks like it might cause trouble, plus more at strategic intervals. Like I said, no problem.