Is semen impure, and what is the ruling on praying in clothes on which there is semen, with knowledge of that?

Semen is pure according to the majority of scholars, who cite the hadith of Aisha (may Allah be pleased with her) that she used to rub it off the garment of the Messenger of Allah (peace and blessings of Allah be upon him). Rubbing does not purify an impurity. However, it is preferable to wash it off the garment because it is considered repulsive, and it has been reported that she sometimes used to wash it.