I'm with you, except for the envelope part - I'd rather lick that any day. But I need to be in a generous mood to consider blowing someone, and it won't be the first thing I do either. Gagging and squirting tears, nose running, horrid taste. I just won't do it unless I'm on top. But as you said, maybe if I really liked the guy and we didn't have issues between us, it would be different.
Though I still do try to puke when they put in the X-ray plates at the dentist's, and I gag if I brush my tongue with a toothbrush etc.