The basic function of a condom is to prevent the transmission of semen. By stopping that one thing, you can prevent both pregnancy and the spread of tons of STDs during intercourse.( like HIV ,Hepatitis, gonorrhoea and chlamydia ).

However, the problem is that not all STDs are spread through semen, meaning a condom may not be the magic shield you thought it was.

Anything that can be spread with skin-to-skin contact is not going to be protected for by a condom,

Here are the STDs that can still be spread even when your partner has things under wraps, and why it's so important to get tested regularly if you're sexually active.

1. Herpes

Herpes are tiny recurrent painful swellings.
If a person has a herpes lesion that is on an area of the groin or pelvis that is not covered by the condom, then transmission from that lesion can occur. Doctors recommend abstaining when you have an active outbreak (that's when you're most infectious) to decrease the risk of spreading the virus to your partner, though it can still be transmitted when there's no visible outbreak, too.

2. HPV

HPV is the most common STD—if you're sexually active, chances are you'll be exposed to it at some point in your life. Some strains can cause genital warts but others don't, which is why many people never know they have it. And even without warts present, HPV can be passed with skin-to-skin contact. Some strains can lead to cervical cancer. IN FACT,MOST CASES OF CERVICAL CANCER ARE CAUSED BY HPV.

3. Molluscum

Molluscum is a virus that manifests as tiny littlebumps on the skin. It's not a very familiar STD, because it doesn't cause any long-term health problems and is typically asymptomatic other than the appearance of bumps. It’s a skin condition commonly transmitted between kids, but in adults you usually see it sexually transmitted. Women will typically notice it on the vulva. The small, round bumps can appear anywhere from two to three months after the time of infection, and may itch or feel tender to the touch or may just spoil the looks of your privates without any symptoms.

4. Pubic lice (crabs)
Pubic lice is not super common anymore, since people groom more now. But if you're rocking a full bush, you do have a chance of catching pubic lice from an infected partner. Since lice lay eggs and live in the hair, a condom will do absolutely nothing to keep you safe.

5. Syphilis

Syphilis presents as a firm, round, and usually painless sore, called a chancre. "Just like herpes, [a condom] is not going to protect you, depending on where the sore is located. Wearing a condom decreases the likelihood it will spread, and can be effective if the chancre is covered by the condom, but if it is located on a part of the privates that's exposed, it can be transmitted.

Bottom line: If you’re sexually active, it’s important to get tested regularly even if you practice safe sex
I guess ,casual sex is no more as informal as it used to but now a serious project