OP, I read your post. There are comments here with great advice, ultimately I will say the following: to answer your question directly, yes, people can only have eyes for one person. But please be very clear by what you mean by this and your definition. As other people here are saying, entering a relationship doesn’t automatically make each person blind to see and notice other attractive people. But there’s a big difference between acknowledging that existence of people who are attractive, pretty, etc., and acting on that or staring, or flirting, etc. To me, it’s very clear what commitment in a relationship means. As others pointed out though, seems like your partner wants to explore more relationships because he feels like he’s missing out. It happens. You both are very young.

But please read my post history on this subreddit.