So that's how it is. My dumbass self thought the baby might drown in a pool of semen or at least choke on it.
To go into further detail... despite what exaggerated hentai visuals would have you believe, in reality semen doesn't actually enter the womb during ejaculation due to the extremely small opening of the cervix when it's closed (it only opens during menstruation and childbirth), plus the constant presence of mucus in the cervix which acts like a seal that prevents air from entering the womb.
When a woman is pregnant or otherwise not close to ovulating, this mucus is thick and prevents the passage of sperm (not including semen which is always left behind in the vagina); it only becomes clear and watery on her "unsafe days" which allow sperm to swim through the cervix and into the womb.
The implication of course, is that impregnation can only happen several hours after sex (this gives women a window of time, like ~72hrs to take emergency "morning after" pills that thicken this cervical mucus to block any more sperm from entering the womb) - so yet again hentai is exaggerating about instant impregnation immediately after sex.
Fetuses are also always covered by a protective layer called an "amniotic sac", think of a thin balloon that contains fluid - when people say a pregnant woman's "water broke" they're referring to this layer breaking when she's close to childbirth.
tl;dr: I agree with what the other guy said, normal sex during pregnancy is safe for both the mother and the baby.