Whoops, sorry, just realized you were a dude.
It isn't a big deal. However, a small percentage of men CAN have allergic reactions to sperm. Their own sperm won't cause any problems when they ejaculate, because the penis has an inner membrane that protects against any potential harmful effects(also a major reason why male urinary tract infections are so rare). But the mucous membranes elsewhere in the body, like your anus for example, are highly susceptible if you happen to have an allergy to it.
An allergic reaction to sperm can feature a sharp, burning pain, but most likely just an itch that can range from mild to severe. Interestingly, for years people thought this pain and discomfort was a result of the act of anal sex itself, the friction of the penis against the anal cavity. Now, while that can of course cause similar pain, recent studies have discovered that in some cases this is in fact due to sperm allergies.
Merely washing out with soap and water isn't going to do the trick, not if the sperm cells have already had time to sink themselves into the cells making up the mucous membrane of your rectal wall(think a sperm cell entering the female egg, sperm are wired to do the same thing even if all that are present are rectal cells).
The only thing that will effectively kill the sperm cells and prevent an allergic reaction, again, if you happen to have this allergy, is to perform an enema with a solution of water, alcohol, and yogurt. The yogurt is optional, somewhat of a folk remedy for this problem, in that it can help sooth the rectum. It's used frequently in nursing homes as an alternative treatment/therapy for hemorrhoids. I've also heard of women and gay men using it after a particularly hard bout of anal sex, or after an encounter with numerous partners in short succession.
You don't want to use too much alcohol(I'm referring to rubbing alcohol here, like you would use to sterilize a cut). It says on the bottle, "Not for internal use", but it will be fine if you mix it with enough water. A solution of 85/15% water to alcohol is what you want to shoot for.
One more helpful tip, do not use fruit at the bottom yogurt, or anything that has large chunks of fruit in it. You don't want things to be more messy then they need to be.
Good luck!
