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I can understand your problem, and I will help you the best possible.
It is impossible for making someone pregnant by masturbating in bathroom. It is not possible for the sperm that is released during masturbation in bathroom to travel up to vagina to fertilise with egg released by the women. Sperm must enter the vagina in order to cause pregnancy. Sperm is alive as long as it is moist. Once it dries up, it is no longer a threat.  So for pregnancy to occur there would have to be live sperm on the toilet seat, but the sperm cannot live very long outside the body.  They prefer warm environments and don’t survive when exposed to air, so they don’t have a very good chance of surviving on a toilet seat, and then it would have to enter the woman's vagina and that woman would have to be extremely fertile.

So In short :

1.The sperm would lose motility / be dead in few minutes in toilet seat.

2. It cannot move up in external vagina (unfavourable environment, no direct access)

 A lot needs to be happen and this ins nearly an impossible thing to happen.

So you absolutely need not worry about pregnancy from masturbating onto the toilet seat.

Hope this helps.  Feel free to ask if you have any further queries.

Thank you.