This article defends the claim that sexual penetration is a prima facie wrong: it requires justification. We defend this claim by reference to considerations relating the use of physical force required to achieve sexual penetration, the occurrence and risk of harm posed by sexual penetration, and the negative social meaning of sexual penetration in patriarchal societies. The step we take in this article is a preliminary part of a larger project. We are not here directly concerned with questions of criminalisation; we aim simply to map the moral landscape of sexual penetration.