I don't see how it's easier to lift her bikini up rather than just take it off. If it's loose enough she can lift it up, then her boobs could fall out any time.
But of course there's no logic here. The only point of this scene is to tempt the main character.
The logic is that Ria knows exactly what she is doing that's why she suggested this. She's intentionally doing this to try and steal Haruka from Suzuka by tempting him, knowing full well that he lusts after her body so he wont refuse. Suzuka knows this as well that's why she declared she wont give him up to anyone