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.