Lena can retort to Ian acting like an asshole about her stalkfap profile. But she's perfectly understanding if Ian can act maturely about it. Can't say i see how this somehow makes Lena a slut. Furthermore, Stalkfap is entirely optional and up to the player, like every single other activity that could give someone idea to call Lena a slut, so there's no basis to claim she's "a slut regardless of what you choose".
You can play Lena so that she doesn't have sex with anyone and doesn't take a single picture. She's then a slut how, exactly?