Your white cells produce antibodies in response to infections yep, and they know to do this because a foregin cell has antigens on it which tell the body that it is infact foreign.
Under normal circumstances, your body therefore does not attack your own cells, because although each cell also has its own antigen, the body realises that it is from your own body so doesn't start an immune response to it.
However, in the testes, the area which produces sperm is completely isolated from the blood (the blood-testes barrier), so the body has never come into contact with your own sperm, so it will see it as foregin if it was to get into the blood. Therefore, when there is severe trauma to the testicles with internal bleeding, this barrier is impaired, and thus CAN lead to fertility problems. Infact, 10% of infertility cases are due to antisperm antibodies.
Therefore, my question is not completely ludicrous..... If you eat your own cum, wouldn't the sperm get into your blood through your stomach? or would it be destroyed due to the acidity.