Polish chłopak can mean either:
– a boy (also used colloquially among adult men, as is "Hey, boys!")
– a boyfriend
"Guy" can be translated as:
– gość, gościu
depending mostly on colloquiality and politeness levels.
While you can refer to your boyfriend as "my guy", this expression can mean different kind of relationships, not only romantic. So mój chłopak wouldn't always mean the same, as it's either a romantic or parental relationship, but you can say mój facet, which can mean romantic or professional relationship, similarly as "my guy" does.
Most Duolingo courses are strict about meanings. If there's a source-language word that means "A or B", it's better to not translate it into a target-language word that means "A or C" if you can use a target-language word that means "A or B" instead.