You will not get any STIs from splashes of urine; this is not how they are transmitted. STIs are passed through sexual contact with someone who has an STI. You will not get an STI from using a toilet.
Washing your hands after using the toilet is a good idea especially if you are using public washrooms as infections like the common cold or flu can be passed through touching surfaces contaminated with these viruses, but STIs are not spread this way.
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