Casey is a page 3 girl, and they covet showmances and famous guys.
Their whole career pretty much comes down to making sure they stay in the spotlight, and a BB showmance is a good way to make money that way, with the gutter press.
Similarly, Jasmine is an attention whore- she's famous, but not for anything good- she's done a sex tape ffs, she'll do anything to stay in the spotlight. Just look at how her eyes lit up when Lee suggested she stay in England and try and get famous here, she's sensing the possibilities.
Both of them probably think, **** it, why not? I'm single and there's a chance I could get some deal out of it, though in Casey's defence, she does seem like a sweet girl, who did fancy Lee and was annoyed when he played her, even if her eye was also on the magazine deals.
To be fair, I don't think Casey is all that either, especially for a model, and I think Jasmine is just the type that will **** pretty much anything that moves, she's pretty crazy, so she probably just doesn't see the big deal.