And that would have made it right – Yichud (not mamesh arayos)versus potentially committing suicide? Even the Torah does not say a boy and girl who have been intimate together are chayav misah (unless she was an arusah or an eishes ish). To the contrary, he is supposed to marry her. And that IS mamesh arayos.
For most of our history, you would have been correct. However, in this day and age, most young women are Niddos, and that in of itself is an ervah (not weighing in on Yichud, which may only be a Takana (depending on Pshat in Avoda Zara 36B)).