To be a child and clueless about human sexuality. She's not doing anything wrong - with the exception of pursuing other people's S/O, which also qualifies the S/O's as being unfaithful *takes 2 to tango. But the Male protagonist is correct that she relates to bonding with sexual pursuits and that's not healthy in the longterm, primarily because if your social circle (with the exception of polyamorous relations) is simply made out of 1 time lovers or, if you pursue sex to feel validated, you'll just end up in a spiral of self-loathing and dopamine fixing.
People who have had sex and are not teenagers will understand.