There's no single answer to whether guys prefer thin or thick lips. Personal preference varies greatly. What one person finds attractive, another may not.
Individual Preferences Reign Supreme
As stated in a 2017 source, "Guys are individuals, not some type of manufactured machine with preset preferences." Attraction is complex and depends on many factors beyond just lip size. Source
Lip Size is One Factor Among Many
While some men may express a preference for thin lips, others may find fuller lips more attractive. A Quora thread discusses this, noting that some men's preference depends on overall facial features and the person's personality. Source Another source mentions that the preference can even vary based on race or ethnicity. Source Furthermore, the focus should be on the entire person's appearance and personality, not solely their lip size. Source
The Role of Cultural Influences
Cultural trends and media portrayals can influence perceptions of beauty. The rise of lip fillers and the popularity of certain celebrities with fuller lips might contribute to the perception that full lips are more desirable. However, this doesn't negate the fact that individual preferences widely vary. Source
Beyond Physical Appearance
Ultimately, attraction is multifaceted. It involves the entire person, including their personality, humor, intelligence, and overall presentation. Focusing on one singular feature like lip thickness oversimplifies the complexities of attraction.