Nathan isn't masturbating

It wasn't long before Dr. Vierre noticed something odd going on in the house, or rather, not going on. Nathan hadn't jerked off once, in all the recordings. It had become more noticeable over time, as his pent up lust had began giving him boners very often. She often saw him get one in the shower, or as morning wood, or even seeing Gabby showing a bit of skin, or seeing something on TV, an ad, or a program featuring scantily clad women.

She realized that watching him had made him too embarrassed to masturbate, so he had just been holding it in.