In the biblical world, betrothal (engagement) was viewed as equal to marriage. Having sexual relations with a betrothed woman who was not a slave brought the same punishment as adultery. The punishment was death (Deuteronomy 22:22; see Leviticus 18:20).
The situation was less clear if the woman was a slave, since she might not be able to refuse. Slaves in Israel had more rights than those in Mesopotamia or Egypt, but they were still not free. They could not punish a slave girl, so the man also went unpunished. Yet, he was guilty in God's eyes and had to sacrifice a ram for his sin.