I've read this description in a few places and it also happens to be my first one:
"Each of the Enneagram's 9 types can be further broken down into one of three primary variants or instincts: The Self-Preservation, Social and Sexual Variants. Everyone contains a certain measure of all three. But in any given individual, one tends to dominate.
Many people originally identify themselves as this type because they have learned that the Sexual types are interested in "one-on-one relationships." But all three instinctual types are interested in one-on-one relationships for different reasons, so this does not distinguish them.
The key element in Sexual types is an intense drive for intimacy and a constant awareness of the "chemistry" between themselves and others. Sexual types are immediately aware of the attraction, or lack thereof, between themselves and other people. Further, while the basis of this instinct is related to sexuality, it is not necessarily about people engaging in the sexual act. There are many people that we are excited to be around for reasons of personal chemistry that we have no intention of "getting involved with." Nonetheless, we might be aware that we feel stimulated in certain people's company and less so in others. The sexual type is constantly moving toward that sense of intense stimulation and intimacy in their relationships and in their activities. They are the most "energized" of the three instinctual types, and tend to be more aggressive, competitive, charged, and emotionally intense than those of the Self-Preservation or Social Variants. Sexual types need to have deep intimacy in their primary relationships or else they remain unsatisfied. They enjoy being intensely involved—even merged—with others, and can become disenchanted with partners who are unable to meet their need for intense energetic union. Losing yourself in a "fusion" of being is the ideal here, and Sexual types are always looking for this state with others and with stimulating objects in their world."