You can’t get pregnant if your boyfriend cleans himself up with a tissue after he ejaculates, you pick up that tissue to throw it away, and a few hours later you go to the bathroom and wipe yourself. Not even if, in all that time, you haven’t washed your hands.
For gods sake, though, if you’re going to go around picking up dirty tissues and throwing them away… WASH YOUR HANDS AFTERWARD. Eww. Better to leave them all over the floor until they dry out, get all crusty, and are no danger to anyone.
Explanation for why you can’t get pregnant from picking up a semen covered Kleenex (and then touching yourself hours later):
It’s hard to get pregnant. The sperm have to make it through the vagina, get up into the cervix, and find an egg. Wiping the outside of your body with toilet paper held by a hand that happens to have been in the general vicinity of semen a few hours earlier just won’t do it. The semen, and the sperm it contains, have to get INSIDE your vagina.