It's kind of obvious if you played the game. Firstly, he's straight up raped her in the game at least twice. She was unconcious the first time, and handcuffed/blindfolded the second time, but that pleasure would still have been broadcast back to her brain through her headset. She has been getting hornier, and Kirito hasn't been banging her. Instead she's been letting Inoda grope her tits, finger her, and giving him handjobs and boobjobs in return. Now after all the 'girlfriend practise' she's realising Kirito is 'less than average', while Inoda is clearly 'larger than average'. Before Inoda's birthday date, she was masturbating and thinking about how that dick would feel in her. She wants it.
Obviously, we don't yet know how that date day goes. But I'm thinking he's just nice to her and they have a nice time out together, likely him on his 'best behaviour' as a manipulation to get her to do things for him later. Or maybe they sneak in a little 'girlfriend practise' somewhere public but out of sight. Regardless they end up back at Inoda's; he convinces her they need to get naked so their wet clothes don't give them a cold, which obviously gets him horny for her.
How they cross the threshold, we don't yet know. But there are a few hints. Firstly, it's his birthday so he'll use that to get special favors. Secondly, she's just pledged to Kirito that she'll move to America with him, so that puts a her in a 'this is temporary and ending soonm' mindset. Third, she's horny for the feeling of that big dick in her guts; that's weakness he can exploit.
I think he'll use the video of Asuna's mother to blackmail the mother for access to Asuna later, given that she's been proven controlling around who her daughter spends her time with. If she doesn't like Kirito, Inoda is even more of an undeserving loser; ordinarily she would not stand for them associating, so he has to use his recording as leverage.