First things first. If the MC DID get with Yuka it would depend on your definition of incest for it to BE incest. If you consider incest to be any family relationship (including things like step-mother, step sister, and two people in the same family despite not ACTUALLY being related.), then yes, it would be. But, unless my memory is failing me once again (haven't played the game in roughly 5 months), Yuka isn't related to the MC through blood, can't remember if she's adopted or what though. So if your definition of incest requires blood relations, then it wouldn't be incest.
"That's like getting upset that your gf had fwbs before you got into a relationship. Makes no sense."
That means "Friends with benefits" right? (actual question)
If so, then congratulations! You've just discovered a new piece of evidence for why people are upset. In the modern day there's a decent amount of drama surrounding things like OnlyFans and people having copious amounts of sex. Most people don't really want their partner to have had say... a thousand ex partners, as that could signify some sort of problems they don't want to deal with. In other words, it's usually seen as a red flag. Nor do they want others to see their romantic partners naked. A vast amount of people that have an OF page, have both copious amounts of sex (sometimes their videos are WITH their romantic partners and sometimes NOT) AND have people pay to see them naked. Both things people aren't too fond of in their partners as they walk the line between some form of NTR and normal exclusivity. And as such, people aren't too fond of dating both current AND ex prostitutes for that exact same reason. They have copious amounts of sex AND people see them naked all the time.
Chika's relationship with the MC is more of a close friend at the time of her H-scene. If the player chooses "yes" THEN it becomes a singular, smaller step closer to NTR as you've at least had sex with her and that COULD advance the relationship, though in this game it won't because it DOES give you more points with her for turning down her offer, most of the people giving the game an EXTREME amount of hate don't completely understand that.