**Core Concept**
Coitus interruptus, also known as withdrawal, is a method of contraception where the male partner withdraws his penis from the vagina before ejaculation to prevent fertilization. This method relies on the timing of ejaculation and the effectiveness of the withdrawal to prevent sperm from entering the cervix.

**Why the Correct Answer is Right**
The effectiveness of coitus interruptus depends on various factors, including the male partner's ability to withdraw in time and the presence of pre-ejaculatory fluid, which may contain sperm. The method is not 100% reliable, as sperm can still be present in the vagina even after withdrawal. The failure rate of coitus interruptus is relatively high, especially with infrequent use or when the male partner has difficulty withdrawing in time.

**Why Each Wrong Option is Incorrect**
**Option A:** Coitus interruptus is a highly effective method of contraception.
* This is incorrect because coitus interruptus has a relatively high failure rate, especially with infrequent use.

**Option B:** Pre-ejaculatory fluid is always sperm-free.
* This is incorrect because pre-ejaculatory fluid, also known as precum, can contain sperm, which can lead to unintended pregnancy.

**Option C:** Coitus interruptus is a barrier method of contraception.
* This is incorrect because coitus interruptus is a withdrawal method, not a barrier method, which involves the use of a physical barrier to prevent sperm from entering the cervix.

**Option D:** Coitus interruptus is 100% effective if used perfectly.
* This is incorrect because even with perfect use, coitus interruptus is not 100% effective due to the presence of pre-ejaculatory fluid and the male partner's ability to withdraw in time.

**Clinical Pearl / High-Yield Fact**
It's essential to note that coitus interruptus is not a reliable method of contraception, and couples should consider using more effective methods, such as hormonal contraceptives or barrier methods, to prevent unintended pregnancy.

**Correct Answer:** A.