Is NKT a sex cult in thin disguise?
No, because Highest Yoga Tantra is not about sex. This is a common misunderstanding about Tantra.
Monks and nuns who are celibate practise Tantra because it's not about sex - it's a special quick path to enlightenment that is the very essence of Buddha's teachings. Anyone with who has true compassion will want to practise Tantra so that they can help all living beings as quickly as possible.
Geshe Kelsang has made Highest Yoga Tantra open to every one to practise, but it still requires an empowerment. Even though the instructions are available widely (probably because something that is secret is more attractive) they don't work unless you receive an empowerment, so the empowerment and instructions are given every couple of years. It's not for the purpose of making money, as Superscot would have us believe, but because it's an integral part of the Kadampa path. However, no one is introduced to Tantra against their will or too early. As the instructions are open (anyone can read a book), people decide for themselves whether they want to practise Tantra or not - there's no coercion and no pressure.