Corrected entry: Charlotte is worried that Trey's inability to perform in bed is physiological, and is relieved to learn that he can have an erection. In a previous episode she gave him a hand job, so she should already know that he can get it up.
Correction: It's psychological, Trey was performing fine (the hand job) until they got married, and then he admitted he saw her as his wife, a pure being, and couldn't achieve an erection.
Correction: In fact, believe it or not, it is possible for a man to be manually masturbated to orgasm by a sexual partner without achieving an erection - hence Charlotte's concern.