If a guy ejaculates into your anus, is it bad?

YES. That means you were engaging in unprotected anal sex, and that's one of the riskiest sexual activities you can do.

The anus and rectum are lined with soft, delicate mucous membranes. Without proper protection and lubrication, these tissues can get torn or injured during anal sex. not only can this be quite uncomfortable, it can be bad for your health.

Can you get pregnant from semen in the rectum? Yes, if/when it dribbles out of your anus and onto your vulva. The sperm can make their way into your vagina and up to your Fallopian tubes in that scenario, and then you can get pregnant.

Can you get STDs? By golly yes you can!!! The anus and the rectum are not clean. Even at their cleanest, they contain traces of feces and loads of bacteria. These germs can get smeared onto your vulva and into your vagina, resulting in some heinous infectious. Chlamydia and gonorrhea can infect the anus, too, resulting in pus-discharge, soreness and painful defecation. You can also infect the anus and rectum with herpes and HPV, resulting in anogenital herpes, or anorectal warts. Worse yet, the same strains of HPV that cause cervical cancer can also cause anorectal cancer.

Oh, and let us not forget that unprotected anal sex resulted in a large proportion of HIV infections in the Western world.

So it's important that you always use a condom with thick water-based lube when engaging in anal sex. For more on that, check out Safe, Sound & Sexy: A Safer Sex How-To