If I recall, Paul was seduced by the maid, that she had made advances towards him and eventually he gave in
I managed to find the light novel, here's the TL;DR: Followed by the longer version.
Paul sexually assaulted Lilia 6 years before she became a maid for him.
A fat noble made advances next, making her see how much better it had been with Paul.
Lilia used that to her advantage in maid hiring negotiations.
Paul hadn't made advances towards Lilia except for the occasional gropes, which she resisted.
Lilia's own needs had grown more and more over the years.
Lilia seduced Paul to satisfy her own needs.
Light Novel; Chapter 9 - Emergency Family Meeting:
Paul hadn't been allowed to have sex with Zenith during her second pregnancy, because of this, his lust had grown more and more, and during this time, he had snuck into Lilia's room in the middle of the night.
The day after the meeting, Lilia is inner monologuing about how she had her needs, and how hearing their moans every night, and cleaning their room that smelled of man, and seeing Paul sword train had all made it too much for her to bear, so she had purposely seduced Paul into having sex with her to relieve her own needs. While she had not intended this when first moving in as a maid, she had always had feelings for Paul and, over time, this had only grown.
She also inner monologues about how, when they were younger and training in a training hall, Paul had snuck into her room at night and taken her by force, that while she didn't dislike him at the time, she certainly didn't love him, and had cried at first. But then a fat noble had made advances on her later, and that put things in perspective of how much things had been better with Paul.
And then 6 years later, when she heard Paul was hiring a maid, she used what he had done to her in her favor during negotiations.
In that time, Paul had really developed into a fine man, according to Lilia's comment "the last 6 years had certainly been kind to him." Paul hadn't made advances towards her in the beginning, though he would sneak in the occasional grope of her, which she could resist at first, but she was fully aware that she was walking a thin line.