Semen in the rectum is not in and of itself dangerous, the feces is much more dangerous and corrosive and your system thrives on it. HOWEVER semen in the rectum presumes unprotected anal sex. Semen can carry almost the full range of STD'd and
Any unprotected sex can subject both participants in the sex act to transmission of STD's.
More than you wanted to know:
well the rectum is the anus leading on to when taking a poop the smegma comes out neon green
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As long as the person does not have a STD, it will not hurt the rectum.
Rectum? Probably not; the rectum is inside the body, and you probably couldn't get to it. The anus? The anus is the ring of muscle that is the exit from the rectum. If he/she maintains some basic hygiene and cleanliness, and isn't ill with ebola or cholera or dysentery, it's probably safe to do. Kinky, perhaps, but safe.
If you swallow your own semen, then no, you can't get anything new. It's quite safe. It's fairly safe, too, to swallow someone else's semen.
It is not safe.
As long as he has no STD's, it is quite safe to consume semen.
Yes it is safe as long as your partner is STD free.
The rifampin in the semen will not cause any problems, but their semen may not be safe to ingest. Rifampin is an antibiotic given for many reasons, including STD's. If the rifampin is being given for an STD then it would not be safe to ingest the semen, not because it has the rifampin in it, but because it may still have infectious particles in it.
no
Yes. It is safe if they are all free of STDs.
Yes it is safe. I do not advise doing that though. It is safe and you can do so if you wish. You cannot give yourself any infection from taking your own semen. Many males masturbate and swallow their own semen. Recent studies have found that consuming human semen reduces depression. You can find an article about that in the related links below.
Yes
Yes. The amounts that might possibly be found in his semen are at such trace concentrations it won't do a thing.
no don't even try.