There were two well-known straight men playing gay characters in the 2013 film 'Behind the Candelabra', which prompted an outcry in some reaches of the Hollywood gay community. By 2013 there was far less stigma attached to being gay than there was in the 90s, and there are plenty of openly gay characters who could have appeared in the lead roles of the Liberace biopic. Despite that, the producers went with Matt Damon and Michael Douglas.
Whatever the reasoning, it was an inspired choice. Douglas was excellent, but the film wouldn't have been the same without the incredible performance of Damon in his supporting role as Scott Thorson, Liberace's assistant and lover. The chemistry between the two was electric and made a success of the film despite the fact that Damon, married to his wife Luciana since 2005, is straight.