Let's look at a homosexual. Say he's a practicing Muslim and for the sake of Allah he holds back his urges and desires all his life and Allah (SWT) rewards him with Jannah.
When he gets to Jannah, will he be able to execute his homosexual desires (it being a sin)? This is just an example. The question isn't primarily about homosexuality, but sins as a whole. Can we commit sins in Jannah?