Semen contains proteins and enzymes along with sperm and water.
The some of these proteins and enzymes react to coagulate(firm up, like a blood clot), some of these enzymes and proteins also cause it to become liquid.
Hot water washes away the bits that cause it to be liquid before it washes away the enzymes that make it solid. Depending how hot it is, it can also denature some of the proteins. Denatured proteins bundle up and get caught on each other like tangling up cords, which create a firmer structure.
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