Oral sex is both a physical pleasure and a symbolic pleasure, and some men really get off on the fact of oral sex more than the actual stimulation they get. The ultimate act in their mind is the swallowing of semen. It is a virtually harmless activity. It would be very hard to get AIDS that way, but you could get gonorrhea of the throat. It is not very fattening, has a lot of protein, and has a salty taste.
Some foods affect the taste and everyone has their individual scent or taste.
If the woman doesn't want the guy to ejaculate in her mouth, then that should be respected. That depends on the culture and what the culture promotes. For example, America has become more sexually enlightened. In the 1950s, the average length of time was three minutes. In the 1970s, the average time was seven minutes, and in the 1990s, the average length of time was nine minutes.