@ZipLine-- It takes a while for itching to go away after scabies is treated. But that's not an STD. I have no idea if the same goes for some STDs. It's best to check with your doctor.
I'm sure itching is also a major problem for those with the herpes simplex type 2 (genital herpes) during breakouts. The best way to avoid those is to take antivirals and keep the immune system strong. I've read that taking the supplement lysine can be helpful. It's an amino acid that supports the immune system. And using anti-itch topical creams on the blisters will help too. Aloe vera is often recommended for itching. It could probably also be used around the genital area (but only on the outside) without problems if it's 100% aloe vera.