Is it not good to cum inside during pregnancy?

Sometimes: Sperm does contain some prostaglandin which is a substance that can stimulated contractions. Sex during pregnancy is usually fine for a normal pregnancy. If there is any risk pre-term labor, bleeding during pregnancy or placenta previa and some other conditions, usually sex is not recommended.

It doesn't matter: Whether or not a male ejaculates or not inside a woman who is pregnant is immaterial. Unless a doctor has said that there's a medical reason not to have sex during a woman's pregnancy, couples can freely have sex. Sometimes women in pregnancy are more sensitive in the vaginal area, so that's something to consider. But it's definitely safe.