Out of context it's worse, but it's still hideous.
In context, it's reference to Perkins having bragged to their circle that after he and [NAME DELETED BECAUSE HE'S STILL ALIVE] got aversion therapy to cure them of their homosexuality, they were "no longer gay."
The aversion therapy, of course, did not work, and Perkins remained promiscuously homosexual for years, despite his protestations to the contrary.
So Laurents's giggles were NOT at Perkins having contracted the disease but at Perkins's hypocrisy being revealed.
Either way, it's hideous behavior but no more or less hideous than anything else Laurents has ever done or said.
NOTE TO DAME: As far as Bernadette goes, I think Arthur has always been careful never to say anything bad about her, just about Sam Mendes and that revival.