If having sex helps Akita, then why not? She was willing to cuddle and to take a bath with him. What's stopping her from having sex with him if it motivates him? As it stands right now, Rikudou doesn't have that reason yet but there might be a time if she sees it fit.
Also no need to think deep for this manga. It's grand and all but logic had been thrown out the window ever since the MMA fight (might be even earlier).
It ties back to why I am kinda confused as to how she fits the story.
Akita's motivator is mostly revenge against the producer and curiosity about violence.
Setsuna here is both mentor and maybe a potential heroine (even if Akita has shown no romantic feelings towards her, only occasional lust).
But the reason why she's sticking to Akita is... Nebulous.
Sometimes she talks like she's doing it only because it's fun or interesting to train a normie into a killing machine, but sometimes her actions make it seem like she's leading him around: living together, the bath, and cuddling.
Doesn't it make it seem like she
wants him to stick around for the training?
Like she's training him with a purpose instead of doing it just because?
Now, a well thought out story would incorporate these elements into a coherent narrative.
As this manga stands, I am willing to bet my breeches she only sticks around and cuddles him occasionally because the author wants big titty crazy jk fanservice.