@TomInHo,
Have you had many of these? First thing I'd wonder is what escalation process you were following.
I have only rarely run into these... a girl who is really resisting, telling me things like, "I can give you a blowjob if you want"... every time that has happened it's been when the escalation was too rushed and the girl was not properly aroused.
The few times I've gotten it I've read it as "she's just trying to be nice, or maybe she wants to defuse this situation that's gotten more sexual than she wants it; but either way I botched the escalation."
My escalation is laser-focused on the pussy though; as soon as I can, my knee is on her pussy, then hand is on her pussy, then hand is down her pants, panties are off, etc. Usually while I'm still fully dressed. By the time my dick even crosses her mind it's because she wants it in her.
If you're escalating some other way, maybe your dick is more front-and-center for her before she's properly warmed up? If so, possible she could be thinking, "Wow, his penis really needs attention, but my pussy isn't in the mood for that. Maybe I can just suck him off."
In which case, if you want to avoid it... perhaps try switching to a pussy-centric escalation instead of a penis-centric one.
(if that advice applies. Maybe it doesn't. It's an interesting thread! I'm curious to see what other responses you get...)
Chase