He says: “I didn’t do it on purpose” as if his lack of intent absolves him.
Is he saying instead: I didn’t think of the consequences? I didn’t think of you? You weren’t in my mind when I flirted? When I trailed my fingertips up her naked forearm to her bicep? When they entwined in her silken blonde hair, when they caressed her nape, when my lips pressed on hers, when I fucked her, when I woke up, when I texted I was staying at Roy’s?
“I was drunk.”
Purposefully. You leave him.