Thank you for that additional information. Given what you have posted here is what I believe is going on and what you should do:
1. You have a fixation with your penis. You should seek professional counseling to help you get over this unhealthy condition.
2. You do not trust your girlfriend. This, too, is not healthy and will make it impossible for the two of you to have a healthy, loving relationship. Time to break up with her if you can not address your issues/concerns about her looking at other men and other dicks. I would suggest that after you call off your relationship you not start another one until you have completed step one above.
Does this make sense to you?
I realize that this was not the answer you were hoping to hear but it would be unfair to you and unprofessional of me were I provide you with anything less than truthful and honest information.
Surely, you prefer that I tell the truth rather than what you wish to hear. Because it reflects very poorly on me unless you press one of the top three feedback choices, keep this in mind when rating my answer and please do not punish me for being honest. I understand that this may not be easy to hear, and I empathize.
Even though my answer was not quite what you were hoping for, please understand that my priority was to provide you with the most honest information. With that in consideration, I hope that you found my answer useful. Please click the excellent feedback button so that I can receive credit for my efforts.
Dr. Rick MD FACS