It's okay. [She knows it's not on purpose and, in truth, it's actually really sweet that he worries he at all! Usually she gets a guilty look or, worse, some weirdly sadistic prideful look. This isn't anything she can't recover from.

But upon realizing that he doesn't quite get her angle:]

I mean, those things are pretty long. And your dick is incredibly long too, so, I mean. I guess it's definitely better if someone else is doing it but ...

[Maybe it wasn't always that long, though? All Nariko is saying is that if she had such an active thing she'd have probably tried to suck it on her own, too. If you need relief then you're gonna do what you can do, man.]