Mhm, I got the "good ending" the first time, so it's probably also linked to your win/loss ratio with her.
(When she was rambling to herself, she had a line in which she said that it should be fine, as you could resist her if you really wanted to)
Getting milked is the final trigger, but it's likely not necessary more than once or twice (as I only took that scene like three times or so).
It's important to get hooked on her venom however. You can use manticore spikes she drops on yourself to help with that