There is nothing in Genesis 3:15 or the surrounding context to indicate the sin of the garden involved sexual relations between Eve and the serpent.

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Polystichum munitum (Wikipedia) Polystichum munitum, the western swordfern, is an evergreen perennial fern native to western North America, where it is one of the most abundant ferns in forested areas. It occurs along the Pacific coast from southeastern Alaska to southern California, and also inland east to southeastern British Columbia, northern Idaho and western Montana, with disjunctive populations in northern British Columbia, Canada; the Black Hills in South Dakota, United States; and Guadalupe Island off of Baja California, Mexico. Western swordfern is known to have locally naturalized in parts of Great Britain and Ireland.

Did Eve Have Sex with Satan? The Serpent Seed View of Genesis 3:15 (equip.org) Branham (1909–65), a proponent of Oneness theology, taught the so-called serpent seed interpretation of Genesis 3, with adherents of this view often citing Genesis 3:15 in support of their position: “And I will put enmity between you [the serpent] and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel” (ESV).