Would you suck her finger after she fingers your ass for a few minutes? Probably not right? Because it will probably smell and taste like poop and it will ABSOLUTELY have poop bacteria on it.

The nice thing about your mouth is that it is pretty capable of handling bad bacteria and even if something goes through you can puke it. The vagina is not nearly as equipped for that and even once making it infected can be a terrible and long experience for the woman.

If you care about her you should never do it unless she understands the consequences and requests it anyway.