That's just what she had to do; he imagines. That bizarre creature who was tied up for three days and found pleasure in the moment he threatened her life. That's just it with Ada, everything is about pleasure for her and that hedonistic attitude is what governs all that she does. Not just sexually, but the way she laughs so freely and brings a light with her everywhere she goes. He's never met anyone that's been through so many hard times and is as happy as she is. This cruel world doesn't appear to have dampened her spirits, except for how hard-headed and determined she can be. 
One thing he does respect her for is that she likes to push people, and see how far they can go. It's that insatiable curiosity of hers that gets her into trouble time and time again. He can hardly criticise her for it, he pushes everyone in this gang too, albeit for different reasons. He wants them all as their best selves, in turn making the gang its best self. Ada's brought a new dynamic with her arrival, for the first time he feels like he's the one being pushed. 
She continues to challenge him, even when she isn't throwing her snarky comments his way and something about that in itself is addicting. Just existing near her, being so close yet unable to touch her whenever he wants is a tough task in itself. Dutch kept his self-control for some time until she worried him sick by going off on a job after he told her no. She's a smart woman, but he'd never forgive himself if he brought her into this camp and let her put herself in more danger. Despite everything, the one thing he wants to do is stick to the promise he made and keep her safe. 
He couldn't contain it any longer. He needed to remind her that he still knows best. After all, if he hadn't have shown her how much she wanted him she'd have never acted on it. She's far too stubborn for her own good. Still, it's not how he would've wanted her lips on his skin for the first time. He succeeded, however, both in opening her up to her own desires and ensuring that she'd follow his rules to keep herself and everyone else safe. 
As he stood there, he thought that if he had her on her knees just once it'd scratch the itch. If he let himself go one time, he'd be satisfied and get to carry on living his life without the diaphanous film over his eyes that taints everything he sees; alerting him if she is, or isn't, right there in front of him. Turns out it was just the tip of the iceberg. 
He knew he was in trouble when he looked down at her as she knelt before him that day. His head was dazed and he couldn't pinpoint what was throbbing more, his chest or his cock. She drives him wild, unleashing this animalistic part of him that burns a hot ache through his entire body unless it's satisfied. But it's not the climax that gets him there, it's seeing that look in her eyes that tells him she feels it too – even if she won't admit it to herself. 
He didn't know it was possible for a person to simultaneously be so defiant and eager to please. The only bound on her independence is that craving for praise, a gentle reminder that she's doing good since she doesn't accept her own judgement as a worthy source. 
Of course, he had to have her again. After seeing her body emerge from that freezing cold lake following her enchantment with the stars above, he knew he wouldn't be satisfied until he felt her, pleased her. And God, was he right. Hearing those little moans he pulled from her while she was wrapped around his fingers had him in raptures. Her laugh is his favourite music but those noises are definitely a close second. 
He had to hear them again, and she gave him just the excuse he needed whether intentionally or unintentionally. That challenging gaze she's designed to provoke him did just that but disparately to how she'd expected. Not wanted, expected. As usual, his desires mirrored hers and reminded her what it was that she really wanted.  
Once again, she flipped it around and he gave her more than he'd planned to. He let her touch him. Gave her that slither of freedom with his body that drove him insane; it's impossible to describe the euphoria of watching her reach her peak as he simultaneously reached his.  
The desire that passed over her eyes can't be ignored: she wanted to kiss him. What's worse, he wanted to kiss her right back. But he can't let himself do that, she'll consume him completely and he's doubtful he'd ever make it back out. Slowly, she's chipping away at his exterior and before he knows it, he'll be whispering sweet nothings in her ear as she rides on his cock. No, don't think about that. 
Regardless of her background, her childhood, and what drove her to pursue outlaws, he has to remember that she was a Pinkerton. She's only part of this gang because she was trying to end it and he cannot let himself fall for her. Sometimes life is cruel as it dangles your deepest desires in front of you for the price of your beliefs and morals. He refuses to be just like those weak corporate men who risk it all for capital gain. He's worked his fingers to the bone and he'd be going back on all his philosophies if he let himself feel for her. 
It appears all his judgements have been shifting since he met Ada. After all, he was the one who loosened those ropes while she slept, allowing her to depart from the gang and live her life. He had no confirmation that she'd keep her silence, but she did. He needs not to think too hard on the circumstances surrounding it, or her reasons for why she chose to let it play out that way. 
Dutch isn't normally wrong, or if he is there's a way to phrase it where he's still technically right, but she disproves that theory regularly. He always thinks he has her figured out, then she surprises him. He looked past her love of good poetry at first, but then he worked her like a mule and she still stood up to give her seat to a pregnant woman she'd never spoken two words to. He thought that sure, she just had a softer side of her that she preferred to keep hidden. Then her talents emerged in playing the guitar, fishing, and even playing poker. All the while acting like it's nothing and keeping that humble nature, someone who does all those things because she truly enjoys them, not because she wants other people to see her in a good light. He envies her for that. He struggles to understand that part of her, her desire to keep those parts of herself hidden. Hell, the woman was even eager to rob and embraced the outlaw lifestyle like it was the first warm breeze following a winter filled with snowstorms. 
Somethings got to give, he's got to try and keep his distance from her. She's clouding every thought he has, infiltrating his mind and causing his train of thought to abandon him whenever he hears her laughing nearby. Her current source of entertainment is Copper, jumping up at her while she drinks a coffee with Arthur. He still has his reservations about the circumstance of their relationship with each other, unfortunately, Arthur is stoic as always and Ada is her usual difficult-to-read self. He doesn't think there's anything between them, but he hopes not and he'll continue to keep watch over their interactions. She's hardly his to claim and he knows that, but he fears she'll only go so long satiating her hunger with the crumbs he gives her. Perhaps two birds can be hit with one stone, if he can get her out of camp and occupy her with jobs it'll give her some satisfaction along with some well needed time in his home without her constant presence. 
He occupies himself with a cigar and waits until the time is right when she's walking past his lodgings. "Miss Ada?" he calls and she walks towards him to lean up against the pole of his tent. 
"Yes Dutch?" 
"I was thinking," he takes an inhale of the cigar, "perhaps it's time for you to join the boys out on a little job." 
Her eyes light up and she looks positively chuffed with herself. "About damn time! What did you have in mind?" 
He can't help but smirk at her eagerness. "Small stuff, for now, homestead robberies and such." 
Her triumphant smile widens, the evening sun casting a glow over her face.  
"Bill mentioned something, I'll check with him and send you out in a few days. You follow their lead though," he points at her as a warning and she lifts her hands in surrender. 
"Sure, sure... whatever you say," she assures with her grin still beaming, pushing herself off the pole to walk out of his sight. 
Dutch thinks of an extra benefit as he watches her leave; it's about time she admits to herself that she wants him instead of prancing around her usual bullshit and leaving him to instigate their encounters.