There is ZERO excuse for it. Disgusting
No, as others have already mentioned, sometimes the buildup of mucus in the nose and fluid in the mouth, particularly on cold mornings, is such that you must expel it, because swallowing is just unhealthy. So long as you consider others when doing it, I see no problem with blowing my nose or spitting during a ride. I used to try using tissues, but found that on some rides, I'd be stopping every 10 minutes or less to use them, and used a lot of tissues, so it just wasn't practical.