Totally safe! Bleach is a very powerful disinfectant and would remove any semen that was on the toilet seat.
Just so you know, semen on a toilet seat is generally not something that could cause harm to another person (or pregnancy). Semen, like other bodily fluids, has to come in contact with genitals or other mucous membranes (like someone's mouth) in order to cause any potential harm due to STIs. That said, it's very thoughtful of you to want to keep the space clean for your family and you did the right thing by cleaning up after yourself.