I must say I feel a bit conflicted about the MC.
On the one hand, he is cheating, plain and simple.
On the other, knowing what the curse does to people (Rikka in beginning of chapter 5), not helping Usui would be a little like seeing a girl get roofied in a club and not doing anything. Then again, "I'm only helping 
her" sounds like a lame excuse for his own gratification.
Having said that, I don't see any way out of this for the MC. Not helping Usui is bad, helping her is bad, and I can't imagine how an honest conversation with Rikka could go: "So hey, you know, your friend also suffers from the curse, so I'll be having oral sex with her as well. You cool with that, right? How about a threesome?" Whatever he does, somebody's gonna get hurt.
Given the mangaka's previous work (
https://mangadex.org/title/616/isuca) I don't think we'll go full Uwakoi (
https://mangadex.org/title/4167/uwakoi). So I'm just gonna switch off my brain and enjoy the smut.