Is it bad if a man ejaculates in a womens anus during anal sex?

Maybe: It is bad if the man is carrying an std, such as hiv, because it could lead to transmission. Otherwise, there is nothing directly toxic to the rectum in semen, and while the semen could potentially leak into the vaginal vault afterwards, there is no absorption of sperm from the rectum.

Anal sex: No. It is not. We don't use bad or good to describe health usually. Anal sex is practiced by millions of humans in many different societies and cultures. In general there are no negative health consequences to this common activity. But, then we should discuss the STD risks, which would be the same if the man ejaculates in the vagina.