system
A chat on subreddit /r/NoStupidQuestions
human
Because he equates a high body count to someone who either cheats or doesn't require the same level of intimacy/closeness in order to have sex. It will mean more to him than to her, and he's worried that it's a frivolous act for her, so she'll have no issues with cheating on him

Also, if she's already slept with all the guys in their friend group, he could be worried that he's just another rung on her ladder. 

$0.02
gpt
The fact that she’s slept with so many within the friend group is a red flag to me. Friendships can be ruined easily when there’s this much co-mingling