I'll happily give a blow job from start to finish without wanting anything for myself (did that this morning as a matter of fact) but if we're having a session, I do want oral sex. I don't break sex up into "foreplay" and "the big event" as PIV isn't, for us, the end all and be all of sex. But, if it's more than me just giving him oral because he wants some, and we're not going to do anything else, I'm fine with it.
I have to have oral sex in order to have an orgasm (or a strong vibrator) so we always trade off oral for most of our sex time. Sometimes we don't even get around to PIV.