It's surprising there haven't been millions of replies already to this - surely there must be loads of experts around! Although more experienced in giving (i.e. cunnilingus) than receiving (fellatio), hopefully these pointers will be of some help.

Is it as gross and dirty as it sounds? Well, putting aside any psychological factors such as pre-conceived notions; it boils down to personal taste. If one's partner is not up to standard on personal hygene it can be unpleasant (I would refuse if it didn't smell clean). With fellatio, prior to ejaculation the only additional taste is the pre-cum (which is produced by the prostate, incidentally) - maybe you could get a taste of some of this from your Master and if you think it too gross then again you've got grounds for refusal (only from experience of my own, but I find it pretty tasteless). Same applies to the sperm - and there's the choice of "pulling out" before he orgasms if you can't stand it. Personal experience has been for partners to prefer to spit it out rather than swallow, and I hope your master would understand if it came to that.

If both of us are std free is there anything to be concerned about? According to the web "experts believe that oral sex without protection is less risky than other kinds of sex", which implies that being std free means no risk.

Is there anyway someone can lern to enjoy this? Personally I very much enjoy giving a partner pleasure, and oral sex can achieve this. If you try it and find you don't like it that could be for a number of reasons - I guess mainly psychological (beyond my expertise, sorry). If you found you didn't enjoy because it was physically unpleasant, such as getting jaw ache, I would recommend practice with a lifelike dildo to try to overcome the obstacle.

Hoping this helps somewhat
VV