Is it permissible to re-wear underwear that was worn during intercourse after performing ghusl for janabah?

The underwear in which a husband had intercourse with his wife is pure (ṭāhir), and there is no harm in wearing it or praying in it, even if some semen got on it, because semen is pure. Evidence for this is what was authentically reported from Aisha, may Allah be pleased with her, that she used to rub semen off the garment of the Messenger of Allah, peace and blessings be upon him. Had it been impure, she would not have sufficed with just rubbing it off.

In summary, the underwear in which a man had intercourse with his wife is pure, and it is not obligatory to change it unless it has been stained by impurities such as madhy (pre-seminal fluid) or the like.