"Sorry, I'm not really into that."

"Sorry," you say, "I'm not really into sucking cock." You continue licking her.
She makes a little groan of sexual frustration. "But I can't cum from my pussy alone. Please, do it for me?"
She has no idea you chose to give her a male organ pleasure preference.

You could relent and blow her, which might please her now, but she'll feel bad if you are only doing it to help her.
You could be firm, tell her you just aren't interested in her male parts, but she says she'll never cum without it. It might be cruel to never pleasure her to orgasm, only leaving her to masturbate if she wants to cum.
Maybe you can think of another idea?