Bionano/biomemetic replicators that use various tricks such as pheromonal flooding, eidectic subliminals, etc to incite pair-bonding among any sapient bionts in the vicinity.
The replicators are actually a form of neogenic microorganism and are spread through kissing, sexual intercourse, co-wupism, and any other behaviour which involves exchange of bodily fluids.
From a technical perspective, Love Goo outbreaks are easily contained, as the replicator is large (usually of the orders of several tens of microns, although much smaller forms (down to virus size) while rarer, are also known) and easily sterilised or destroyed. However many infected societies or sub-cultures, having been "blissed out" and feeling a "golden glow", strenuously reject attempts at "cure". Forced inoculation may results in suicide, vandalism, or other dysfunctional forms of behaviour, and require extended and costly treatments in psychonanotherapy before the sophont in question can once again be a useful member of society.
While crude forms of Love Goo date back to the early interplanetary age, most currently active Love Goo is based around Erotocracy, Zoeific, or Disarchy templates.