Thanks for your question! A large penis puts pressure on your bowels, which (as you have experienced) can cause diarrhea. Some doctors suggest relieving your bowels naturally, or taking an enema before having sex.
Enemas can be harmful, so please consult a doctor before use. Loose stools can also occur from any number of diseases or infections. Please visit your doctor to rule out a medical issue, such as irritable bowel syndrome.
Diarrhea is also included as a side effect of Junel, which may be the root of your problem. Consider consulting your doctor about a different birth control option. Anytime you have diarrhea for more than 24 hours, you are at risk of severe dehydration and should seek medical attention.
I hope this helps! Please check back and let us know how you are doing!