Sacred Sexual Union
Tantrics see the Divine in all aspects of the Universe. They understand the Universe, and therefore the Divine, in terms of interacting opposites - yin/yang, active/passive, emissive/receptive, masculine/feminine, powerful/motionless, and so on. Sacred sexual union represents the union of opposites, in which all the apparent contradictions of all opposing pairs are resolved in unity.
The term "sacred sexual union" does not refer to Tantric sex, or to Tantric rituals. The use of actual sexual intercourse in Tantric rituals is symbolic of sacred sexual union, in which all duality is dissolved, but it is just a symbol, not the actuality.
Divine Masculine and Divine Feminine
The Divine Feminine is a representation of one side of each of the infinite number of pairs of opposites. The Divine Feminine cannot exist without the other poles, so the existence of the Divine Feminine requires, also, the existence of the Divine Masculine.
The separation of the Divine into pairs of opposing forces is the first step of illusion, underlying the creation of all the other layers of illusion which prevent us from perceiving the Universe as it truly is - a unified whole.
The sexual union of the Divine Feminine and the Divine Masculine represents the underlying oneness of the Universe.