Ngl Sayaka is kinda annoying, they're not related by blood, if they wanna mess around with each other or get intimate, that's none of her or anyone else's business
MC is being kinda dumb, why would you assume that she's interested in other guys when she's been literally attached to you since y'all were literal babies, she's stated countless times that she loves you and only you
So why tf are you so surprised that she rejected that side character? Come on now, don't be so dense my guy