(Most of the Siddha yogi books are badly translated. but this edition I have depicted above is one of the best translated texts, with elegant English that conveys the difficult topic it translates)
Tantric yoga is misunderstood. It is a hugely spiritual sadhana which uses the unusual path of sublimating the spirit and soul through the body. It starts off with the basic premise that you use this practice for intense self-control, then direct that controlled energy upwards, towards a higher goal.
Some of the most wonderful books on this subject are Tirumandiram, a Tamil text by Siddha yogis which lists practices and discusses the spiritual upliftment that comes from controlling or redirecting the sexual energy.
One of the benefits/offshoots of this sort of yogic control is vitality, vigor, and superior control, including at the uro-genital system. This is why perhaps there is the belief that it is used only for sexual purposes, that would be to undermine the relevance of a spiritual practice.
Any of the yogic practices may be used, as a sequence, to improve fertility, depending on the body type and capacity of the practitioner.
Bandhas play a major role in this, and must be introduced by and by. The vajroli mudra and ashwini mudra are used to prevent decay of the body from the pelvic region upwards, and used to strengthen the entire system. Moolabandha called the master lock combines these, plus adds its own tightening of the pelvic region. The west has reinvented these as Kegel's exercises, as these have now popularly known.
Having a regular practice is more important than focusing on just a few asanas towards libido. It is understood that blood circulation in the uro-genital system needs to be facilitated to improve libido, or fertility.
Some contemporary books that help with this topic are Yoga for sex, by Vimla Lalvani. And Pink Yoga, by CP Sharma. The former lists poses that may be done to improve the whole system, as well as gives partnering poses,while the latter book discusses simpler poses/practices you may do on your own.
Interestingly the same practices which help fertility are also used to control sexual desires and develop dispassion and control lust.
(The above is from a column I wrote a few years ago, on this topic)