First, a cult in the proper sense is simply a group of people who follow the practices of a single teacher. Cults are not inherently bad, and in fact most established faiths are technically cults to the extent that they follow the singular teachings of someone like Gautama Buddha, Jesus Christ, Moses, Muhammad, etc. However, cults can at times form around poisonous leaders who present themselves as wise or enlightened. One must suspend disbelief in matters of faith, but one should not abandon it entirely...
Tantra in the modern (lay) sense is a system generally holds that sexual energy can be used as a tool in spiritual development. Technically speaking, tantra is any system that tries to weave esoteric teachings and practices into common, everyday life (not separate out esoteric life into monastic settings), but because of the emotional power of sexuality in everyday life, tantra has gradually been subsumed by sexuality in the Western world. At any rate, tantrics take sex far more seriously than most people, and it's rare that you'll find a (proper) Tantric teacher who indulges in sex with disciples, acolytes, or lay practitioners. Sex is not 'casual' for them, and their goal is not sex in itself but the ecstatic state that one can achieve by the cultured flow of sexual energy.
Of course in the Buddhist sense, sex is a dangerous fixation. Most Buddhists on the high (monastic) path renounce sex because it is too subject to tanhā. The monastic life becomes a world distinct from the ordinary world, in which higher states of being are cultivated. Tantric traditions have survived in Tibetan Buddhism, primarily in deity and energy yogas. Tibetan Buddhists accept sexual practices as well, though not without consequences (see the recent troubles in the Shambhala Movement). Most modern sexual Tantra derives from Indian/Hindu traditions, which have always carried a more congenial (if complicated) relationship to sexuality (through Shiva/Shakti deity practices). But in the West, with the West's typically prudish-prurient attitudes towards sex, poisonous teachers are rife. One needs to take care.