Since I haven't done any hard research on swallowing semen, I'm affraid this is all theory :p
I'm assuming by 'sperm poisoning' you mean 'semen poisoning'. I couldn't forsee the sperm themselves doing an lasting harm in the small doses you gain through a single ejaculation. Being biologically fragile, they're easily digested in the stomach. The most I think I could say about them is they're high in protein!
The seminal fluid itself is kinda a nutritional soup designed for the sperms health and well-being on their one-way trip. It contains all the sugars, amino acids, vitamins and minerals needed for the cells to run optimally. I wouldn't go so far as to say you could live exclusively on the nutrients in your cum, but it's damn healthy stuff! (No, I'm not kidding). I get sketchy on the details but I know it also contains hormones, some mucus and will almost certainly have trace amounts of urine.
If I was to imagine a way of being poisoned by semen, the best I can come up with is 'zinc poisoning' - since it's in very high conentrations comparitive to everything else. I don't want to imagine how much jizz you'd have to eat to try this one, but it'd be ridiculous. And likely impossible to obtain without several men working shifts on a bucket. Oh the roughness.
In fact, the amounts of ANYTHING we're talking about in semen are so small, that if you could successfully pump it out of someones stomach - You deserve a god damn medal in the field of keyhole surgery. Let's face it, unless you have balls the size of your head the amount of semen you'll produce won't fill a tablespoon let alone someone's stomach. Wherever you heard this, it's likely bullshit. Semen is non-toxic, human-friendly and even highly nutritional.
Enjoy!
Edit: As a final thought, it's entirely possible to be allergic to Semen. However, this would also show when any part of the skin comes into contact with semen and cause fairly obvious reddish inflamation. Such allergies can be permenant (serious), one-off (skin irritation) or seasonal (susceptibility caused by another factor inciting oversensitivity) depending on their nature. I've had it happen to one or two girls who were a little over-sensitive, but no reaction bad enough that they'd need medical treatment or even have a mark that lasted more than a few hours.
Yhorian
Edited 2 time(s). Last edit at 12/09/2007 01:01AM by yhorian.