How can sticky semen be removed from clothing, given that it leaves a white stain, and that the two methods mentioned in the Sunnah (washing it while wet and rubbing it off when dry) are not sufficient to remove it?

Semen is pure according to the preponderant opinion of scholars. It is recommended to wash it when wet and to rub it off when dry. If its trace is not removed by rubbing, it should be washed to avoid divergence of opinion. Based on its purity, the persistence of its trace, whether wet or dry, does not cause any harm.