I don't find it gross or dirty at all. I have to admit though, that at first I didn't care for it (giving head) but now it's gotten so that I get a lot of pleasure from it, in addition to getting pleasure from pleasing him.

One thing that may help you learn to enjoy it more is to have a vibrator on yourself at the same time as you are going down on him. And if your jaw gets tired, take a break and use your tongue for a bit. Ask him to give you directions on which things he especially likes and which things he wants you to change as you're doing it, this way you can learn what he wants.