Thank you for your inquiry. From what we gather from the question, you were asking about the possibility of HIV transmission in the event that you had sat on HIV positive fluid, and it had come into contact with the head of your penis, through your underwear.
From the information given, this scenario is determined to be of No Risk (HIV transmission is not possible in the given scenario).
The scenario you have described does not meet the requirements for the HIV Transmission Equation. The equation is not fulfilled because the scenario above does not involve direct access to the bloodstream through a High, Low, or Negligible Risk activity.
Transmission of HIV must involve human to human interaction, and it cannot be acquired through inanimate objects or insects. For this reason alone, you are not at risk of acquiring HIV.
Recommendation: No need for HIV screening, please refer to health care provider for additional health related questions.
All the best,
AIDS Vancouver Online/Helpline, Cody