Bionanotech / biomemetic replicators that use various tricks such as pheromonal flooding, eidetic subliminals, and other tricks to incite courting or sexual activity among any sapient bionts in the vicinity who have that capability. The replicators are actually a form of neogenic microorganism. They may be spread through kissing, sexual intercourse, co-wupism, and any other behaviour that results in exchange of bodily fluids.
From a technical perspective, Love Goo outbreaks are easily contained, as the replicator is relatively large, usually of the orders of several tens of microns, (though forms down to virus size are known) and once detected and identified is easily sterilised or destroyed by natural or automatic immune systems or by bionanotech Blue Goo. A complication is that if the infection takes hold and spreads there may be whole societies or sub-cultures of individuals who, having been "blissed out" and feeling a "golden glow", strenuously reject attempts at "cure". Forced inoculation of these unwilling subjects may result in suicide, vandalism, or other dysfunctional forms of behaviour, and they may require extended and costly additional therapy before they can once again be a useful member of society. This is one of the aspects of the infection that distinguishes it from many of the other kinds of mind-altering goo such as infectious agents in the Disinhibition Plagues, or the several varieties of goo that cause chronic anger, anxiety, or depression.
Crude forms of Love Goo, some of them recreational, date back to the early Interplanetary Age, and so many designs have been developed since. Most currently active Love Goo is based around Dionysian Erotocracy, Zoeific, or Disarchy templates.