The doctor realizes that Sebastian and his mother used to go everywhere together and as trendy jetsetters, they surrounded themselves with the same type of people. When his mother had a stroke, he starts going with Catherine on trips. She realizes that he likes men though it is never discussed and to say so seems like an insult to his character.
It turns out that Sebastian had used his own mother and his cousin, Catherine, like bait, to lure in men he thought were attractive and then he used them for sex. While in Spain, Catherine and Sebastian were being harassed by a group of men begging for money. When Sebastian rejects their please, he takes off running and while Catherine tries to catch up to him, she finds that he has been attacked by the boys and eat him so like he was a man eater by finding men and spitting them out when he was through, the men that literally ate him, devoured him. This is common of Classic Hollywood because being homosexual was taboo. Sometimes there were even depictions of violence even though it was sometimes just playful. It is not surprising that a homosexual was basically ‘preyed’