Kissing or other intimate public displays of affection (PDA) while wearing a military uniform is generally prohibited and considered a violation of military dress and appearance regulations. The uniform is a symbol of the nation and the service, and professional decorum is required while wearing it.

Key Regulations and Principles:

Professionalism: Uniform wearers are expected to maintain a professional, dignified image. Excessive PDA is viewed as unprofessional and can be subject to disciplinary action under the Uniform Code of Military Justice (UCMJ) for conduct unbecoming an officer or service member.
Specific Service Rules: Each branch has its own regulations (e.g., AR 670-1 for the Army, AFI 36-2903 for the Air Force) that govern conduct in uniform. These rules typically prohibit behavior that could bring discredit upon the armed forces.
Context Matters: A brief, discreet kiss (such as a peck during a homecoming or farewell at an airport) is often tolerated. However, prolonged, intimate, or overtly sexual contact in public is unacceptable.
Location is Key: PDA in uniform is most strictly prohibited while on duty, in a military formation, or inside a military installation. Civilians should also be mindful of these standards when interacting with a service member in uniform.

For context on other serious matters governed by military conduct codes, you can read about What are the five types of military discharges?.

For the official, external regulations on Army uniform wear and appearance, you can review Army Regulation 670-1.