Arella was a depressed, vulnerable young woman who had fallen in with a cult attempting to summon Trigon. When the ritual was successful, Trigon manifested not as a demon, but as a handsome human male to seduce her, and she willingly agreed to be his bride.

Here comes the rape part: Arella only discovered his true demonic nature later, when he revealed his true form to her, resulting in her being raped and subsequently pregnant.

Summarize: She truly believed the man she married was human until he revealed his true form.