Usually it is not possible to penetrate inside a body cavity without cutting through protective tissues. However, sperm reach the egg via the vagina, uterus, and fallopian tubes (oviducts). Without crossing any epidermal barrier, the sperm can find itself inside which body cavity?

Pelvic body cavity

Explanation:

There are four basic body cavities

The dorsal, thoracic, abdominal and the pelvic body cavity. For the question, the sperm can find itself inside the pelvic body cavity, the pelvic body cavity encapsulates the reproductive organs and the bladder. The reproductive organs then includes the vagina, uterus, and fallopian tubes (oviducts) etc. Thus, the sperm can pass through organs of the reproductive system in the pelvic body cavity.