Under certain circumstances pre-cum can push out some sperm from his penis. For the most part, pre-cum does not contain sperm. The problem is however not pre-cum, but cum. Sometimes a man will ejaculate without wanting to. Some techniques men use to extend the to study an have sex also leads to semen leaking. You can just make a sound or touch him in a way he likes and he cannot help himself but ejaculate.
Why would you take a chance like that? You have no control over it and you end up with the mess.
Please be more careful. It is your future and health that are at stake.