I've never noticed any hesitations on just giving a woman oral sex after someone else has done the same for her, or even after she's been penetrated by someone else, but I think there is a lot of "it depends" involved too. Giving a woman oral simultaneous with penetration or after he's cum in her if it's bareback sex, i.e. creampie, then I think that's a bridge too far for many guys.
On all the scenarios, I suspect it's a subjective mental thing on how much "separation" there is between the male and female. If her pussy had a cock in it 10 minutes ago, then not a problem. If he just pulled out two seconds ago, then problem. Where that line gets drawn, if at all, just depends on the individual and their preferences.