From OpenCog

The SexNode is a type of LexicalNode used to in the mating of some kinds of Connectors. It is intended to indicate connector polarity, e.g. when connectors have a to/from, head/dependent, parent/child, male/female, +/-, cation/anion type of polarity that is associated with them.

Sex need not be bipolar; there might be only one Sex, or more than two kinds of Sexes, with specific mating rules. The mono-sex type is usually denoted by a *, meaning that there is only one sex, and so mating is unconstrained.

See Connectors and Sections for details.