It is not any particular sentence, but Duo does not seem to be consistent in deciding whether the preferred translation of φίλος is boyfriend or just friend. And similarly, whether φίλη is a girlfriend or just a (female?) friend. So, I am confused here. Does φίλος tend to get used when it is unclear what sex the friend happens to be?
Quite the opposite. Using "ο φίλος" or "η φίλη" show exactly which sex it is. A friend who is male is "Ο φίλος" while the female is "η φίλη". Whether the relationship is purely friendly or more romantic we then us "boyfriend" "girlfriend". For all sentences containing "αγόρι" we give "boy" and "boyfriend". In this case "ο φιλος" can be either a male "friend" or a "boyfriend".
I would never use φίλος for describing my boyfriend. That's only confusing for everyone who asked OR people then just think "Ah, he's just a friend". ο φίλος μου - my (male) friend το αγόρι μου - my boyfriend