When you do either, your partner's semen will ooze out. However, once you are relaxed it will need to be expelled and either start to leak out of the anus as it does the vagina, or more commonly, you will feel as if you need to either use the bathroom or pass gas.

Ejaculate will likely not come out of the anus immediately following anal sex due to the anal sphinctors as previous posters have pointed out.

The semen will come out, but it depends on two scientific factors known as gravity and relaxation.