90% of the comments here just straight up do not understand this scene, this isnt like him killing the bandits where he accepted the morals of the world and that he gas to adapt in order to survive & get what he wants. This is a 17 year old hormonal loner virgin with no experience besides maybe hentai, he really likes her and desperately wants to have sex, but he also wants to have consent while doing it and for her to also enjoy the act. He also tries to treat her like an equal human being instead of a slave but isnt very good at actually doing so due to his lack of communication skills and experience
MC is being rational and trying to hold on to his morals but basically forgets that she doesn't have the same norms that he does
He holds back his lust since he knows that she can't refuse his orders so he tries to give her time to get to know him and to let her know that her opinions will be respected during the day but its more of an imitation of a modern day date from someone who doesnt know how a date works than anything else, it does help, but it doesnt really get the message across that she is allowed to say no to him.
MC is genuinely doing his best to hold back his lust, be respectful and treat her as an equal, (he even tried to get seperate beds) by talking to her, letting her do what she wants to/needs to do in their room, eating together,etc. despite his obvious hornyness (like when he tries to look away while she's changing). He eases her into the intimacy as best as he can by having them wash each other (although his lust wins out during that), then moving on to more and more intimate body parts like her ears and tail and asking for consent each time he does so (this scene is probably him imitating a wholesome hentai he watched ngl) until he can't hold back anymore and kisses her.
Roxanne pretty obviously doesnt actually want to have sex but sees it as something she has to do since he owns her, MC is too blinded by his lack of experience and lust to notice this
You can tell based on his inner monologue while kissing her that he's unsure whether she's actually okay with it and is actively trying to find any sign of resistance or lack of it as confirmation, he takes her lack of resistance as a sign that she's atleast somewhat ok with it (not how that works but he wouldnt know) and slowly interprets the fact that she's trying to please him as consent (would maybe work if they were in an equal relationship but he basically forgot the fact that she's his slave). His lust has completely taken over by this point, he would still stop if she ever told him to, but he doesnt realize that the main reason she isnt telling him to stop is because she is a slave and not because she's okay with it.
MC was genuinely trying his best, he tried hold back his lust and to respect her wishes and only get intimate with her consent, but due to his lack of experience and hornyness and he doesnt realize that she's only "consenting" because she doesn't realize she's allowed to say no.
TL;DR: its basically unintentional rape, he's trying to let her choose and only do it if she wants to but she doesn't realize that she can say no