Should just wash out with regular soap and water. If not, use a protein based agent on the stain to remove it. If spray and wash won't work that is. Or....apply laundry detergent directly to the stain if liquid soap, if powder, make a paste and apply to stain and then rub with an old soft bristled tooth brush (dispose of brush afterwards)...then wash as usual...But seriously, should come out in the regular wash.

That is funny "sperm stains". I think they are probably better know as semen stains. As with any sort of stain caused by a bodily fluid, it's best to clean the stain as soon as possible, preferably while it is still wet. However, if you can't get to the stain until first thing in the morning, you should still have a chance at salvaging your undies. Try mixing one-half teaspoon of dishwashing soap or fine-fabric detergent into one cup (8 oz.) of warm water. Apply a small amount, blotting until the stain is removed. Since your stain is dry this might take a while, but it should at least get most of the yellow tinge off of your boxers. Rinse your underwear with cold water once you are satisfied with how they look. If the stain persists, try mixing one part white vinegar and two parts water, applying a small amount and blotting — but do this only once. Finally, put your underwear through a cycle in the washer and dryer. Of course, you can avoid this hassle just by masturbating with some tissues or a towel nearby. That way you can mop up your semen right away, without having to get up from your bed, preserving your underwear in the process.

They are called wet spots, not stains. Use a towel so you don't have one.

Just wash them.