If we are going for translation of meaning, I think "They might be men" is a reasonable English way to express "Forse sono uomini".
But I expect Duolingo has probably classified the English "might" as indicative of the subjunctive mood. And so they would have it in their system that you must translate "they might be men" into "loro siano uomini" (the subjunctive is typically not covered until you get a long way into Italian).
Just a thought. Take it with a pinch of salt since I'm not on the official team or anything like that.
The only problem is that Duo often has chosen one answer as correct and the robot doesn't accept other answers. So, if you want to help update the robot you need to report it on the Report a Problem or Support options. See the link for more ideas on using Duo: http://duolingo.wikia.com/wiki/Duolingo_Wiki#Getting_Help
Another thing I've learned. When you look at the hints shown under the word ALWAYS use the first word.
"Uomini" is "men" i.e. plural. So "sono" has to be "they are men" since "I am men" would not work. Look for clues in context with the nouns or adjectives that follow the verb.
"Sono una donna." = I am a woman. "Sono donne." = They are women.
"Sono pazzo." = I am crazy. "Sono pazzi" = They are crazy.
I strongly doubt Forse sono uomini is a common phrase...
However forse is a common word/concept that they probably want to teach at this stage. And they have to teach forse to us using vocabulary that we already know (not many choices at this point - considering you can start this unit after only doing basics 1). Hence sentences like this.
While enjoying your holiday in Rome, you are returning to your hotel after visiting the Vatican so you can shower and change your clothes before dinner. Turning down a backstreet, you suddenly stop in your tracks when you see a young boy slumped against a wall crying.
Placing a reassuring hand on the boy's shoulder you ask him what's wrong. "My new bicycle," he says between deep sobs, "they took it! I'd saved for months to buy it and ... and they just took it from me"
"Who took it?" you enquire, patting the boy on the back. He raises a quivering hand and points to a group of young ne'er-do-wells at the end of the street, one of them holding a shiny red mountain bike, far to small for him to use.
"Those men. They took it" the boy sobs. You stand up and your eyes narrow, focusing on the hoodlums.
"Maybe they are men," you say to the distraught boy, rolling up your sleeves and marching towards the thugs, "we'll soon find out..."