I read what your anxiety is over and as I'm sure you've been told on each of the forums you wrote that fear on, I say with kindness, that I'm sure you've been told that is ridiculous. That would not result in pregnancy in any way, shape or form. You didn't impregnate your mother by masturbating in a bath tub! Nope, never.
So, what is the REAL problem? Anxiety, obsessive thought. Don't psychoanalyze yourself and discuss 'catastrophizing' and all of that. That's not helpful. What IS helpful is reaching out to a professional for help. Therapy with a real therapist. A doctor as a place to start. When something is not a rational thought and you won't let it go, then it is time to begin to look at the root cause. It's not what you fear, it's why you fear it. Because anxiety is in the driver's sear of your life. Get help and kick it out of the car! good luck
Meditation can slowly reduce fear. It will shrink the amygdala and strengthen the anterior cingulate.
There are other mind training exercises which can over time slowly help too. (Neurolinguistic training? CBT, DBT.)
Also possibly join support groups. Being around people can unconsciously assist in reducing anxiety. (Brain thinks: "I'm with my tribe, I'm safe, everything is OK." Brain relaxes. Doesn't matter what the support group is or what they talk about.)
Brothers look alike, that is what genetics is about. I have a little sister who people think is my twin, and she is 6 years younger.
I really like del677's suggestions, and specialmom's to see a therapist (if you can afford one). This is really all on you to solve, not just for this time but for getting out of the pattern of having unrealistic obsessive thoughts.