Thanks for the query.
The possibility of fresh semen over toilet seat which can cause pregnancy is minimal as the sperms when coming in contact with fomites like toilet seat cause sperm fractures and make them immotile and hence non-functional, but again if the gap of time interval between ejaculation and toilet seat reuse by a female partner is short, then possibilities do exist that the sperm may get smeared near vagina and if at all it is able to travel all the way up to the uterus from there, then pregnancy may occur. So, practically its a far fetched idea and possibilities are rare.
Hope this helps.
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