I translated a small part of the Experience in Spiritual Practice (साधना के अनुभव), the book by Rameshchandra Sharma. I think this information will be useful for many practitioners to understand the maithuna topic in Tantrism. It is necessary to practice the maithuna ritual when the sexual energy stagnates in the area of the mūlādhāra and svādhiṣṭhāna cakras. When the kuṇḍalinī ascends, the practice with it becomes a form of yoga, which is mainly contemplative. Of course, these boundaries are very relative, however, there is a general principle of connecting the two poles in order to sublimate our vision and energy. Here is the text:

The Fifth of the Five Makaras

In tantric sādhanā, the practice of pañcamakāra is found. Practice with madya (wine), māmsa (meat), etc., it can also be done with their substitutes. In the pañcamakāra, the fifth is associated with the sense-oriented pūjā, which is known as maithuna. In essence it leads to the regulation of the semen. Maithuna is also called “lata–sādhanā“, it is a special practice for awakening the svādhiṣṭhāna–cakra.

Often after the awakening of the kuṇḍalinī, when vāyu (energy) rises to the svādhiṣṭhāna–cakra, kāma (passion), ascending there, begins to transform into what we know as yoga–sādhanā. Through the identification of a woman with Devī (Goddess) and himself with Śiva, the practitioner leaves the gross states of the physical body, and his spirit and consciousness are directed to the sphere of spiritual consciousness. This often happens when practicing a mantra with prāṇāyāma. The power in the svādhiṣṭhāna or maṇipūra–cakra begins to intensify and the practitioner through asceticism becomes an ūrdhvareta – one, whose ascending sexual power is sublimated. Near the maṇipūra–cakra (just below it) in a woman is the womb (garbhāśaya), which is the most important area in their body. 

So, from the svādhiṣṭhāna–cakra to the maṇipūra–cakra there is a point of two poles, the connection of the spiritual and the physical. and through the practice of japa you connect these centers in a pure sāttvic state. In a state of full bloom of power, at the beginning of the practice, you use a special method, thanks to which the power increases and moves further in the direction of the anāhata–cakra. Mental worship is the essence, thanks to which everything basic happens there.

In the daily pūjā you can invoke the Goddess into a red flower (yellow in the case of Bagalāmukhī), and invoke Śiva in a white flower. Place the flower where the Goddess is on top of the white flower of Śiva, that will be the symbol of viparīta-rati, symbolically connecting Śiva and Śakti.