Thank you for asking your query at DoctorSpring.
Since you haven’t mentioned any other symptoms I am going to assume that you do not have any symptoms such as discharge from the penis, painful urination, fever or ulcer over the genitalia. So I can come to the conclusion that the red bumps and rashes with itching following a sexual intercourse is an STD and most probable cause being early genital warts caused by HPV (a Virus). A fungal infection or HSV infection (Herpes Infection) is also possible. Unfortunately without STD tests it is not possible to comment on the exact cause. (And that the exact reason why these tests exists !) The appearances of individual disease overlap a lot. The treatment is also different for each cause. (Viral / Fungal).
Dermatitis due to friction (skin irritation) can also appear like this. But in that case ‘itching’ is very rare. So the most probable reason for your condition is an STD, for which you need to get tested. These tests are very simple and treatment is of short duration. You may also want to inform your partner about this so that she can also get tested.
Meanwhile I have the following suggestions:
1. You can wash your penile area with slightly warm water.
2. Do not use harsh soaps or detergents or antiseptics to clean
3. Make sure you are wearing a soft cotton undergarments
4. Using Condoms will prevent you from STDs and AIDS. So consider this option every time when you have sex with your partner
Hope this helps