The word sex is used in two ways on this site:
When describing using the internet to meet people for sex, the word sex means any sexual activity between two or more people.
When describing assumptions made about people’s gender based on their body, sex refers to the male/female physical development. In an infant, the sex is judged entirely on genital appearance at birth. Other factors, such as chromosomal configuration, are seldom tested unless a genital anomaly is present. There is an assumption that an apparently male infant will identify as a boy and vice versa.