 A related, influential argument focuses on the definition of marriage. This argues that marriage is necessarily between one man and one woman because it involves a comprehensive union between spouses, a unity of lives, minds, and bodies. Organic bodily union requires being united for a biological purpose, in a procreative-type act (Girgis, et al., 2010). Like the new natural law arguments, this has raised questions as to why only, and all, different-sex couples, even infertile ones, can partake in procreative-type acts, and why bodily union has special significance (Arroyo 2018, Johnson 2013).