I swear people lack a fundamental grasp of the concept of the show and remove all context of being on LI. If you’re not lucky enough to find an ideal partner straight away, you have to settle with the best available option until someone you genuinely like comes along. Him choosing Shaughna over Rebecca doesn’t mean he liked her, it just meant he still deemed her as the ‘best of a bad bunch’. So to speak.
Callum is not the first or only person to do this. The only reason why this situation is such a thing is because Shaughna gave more of her heart to Callum than she should have, considering he offered no reassurances and didn’t appear to be investing himself emotionally into her. She saw everything coming but she acted on hope rather than reality. Everyone has a responsibility to protect theirselves to some extent, and she did a very poor job of that.