MIght want to check the walkthrough to be sure, but iirc in one of scenes where Lena offers Ian to give him a handjob, she mentions his cock is one of nicest she knows. One of responses Ian can give is "
One of the best?" which starts a path where Lena starts sharing stories of her earlier partners with him. You can escalate this further with more of the sad face options during their later interactions.
If you enter the cuck path then Lena can keep denying Ian sex, limiting it to just handjobs and such.