Well, you really don't have anything to fear as long as you don't have intercourse (penis into the vagina where there is an ejaculation inside the vagina) or an ejaculation by your boyfriend at the opening of your vagina.
Sperm is actually very fragile. It can't live even a few seconds without the protection of semen, which is the whitish fluid that it lives in. It can't live in the air at all, and wouldn't survive for any reason on a toilet seat. On a toilet seat it means that a guy would have to ejaculate on the toilet seat, and he doesn't do that. Even if he stands up, urinates on the seat, there is no sperm in urine and there can't be both urination and ejaculation at the same time.
Sperm can live for 1/2-4 hours INSIDE THE reproductive tract with plenty of semen to keep it swimming and alive. It lives for not even a few seconds outside. Even if a man gets semen on his hand from masturbating and then puts his fingers into the vagina there is little sperm that seems to be able to survive.
Males produce millions of sperm daily because they need that many since so many die off and can't survive, swim or otherwise find their way to the egg released inside the female body. The sperm and egg must fertilize way up in the fallopian tube as well or no pregnancy will result either. That means sperm has to travel a long way in order to get to the egg, and most die off.
Wearing a condom over the penis Or inside the vagina) also helps keep sperm from going anywhere as well. It dies very quickly.
I hope this eases your concern, and if you want more information or have more questions you will write me back again. I will gladly continue answering what every questions you may have on this. You don't have to worry, and you can relax with your boyfriend. His sperm can't get to you except in the ways I described above.