I decided to write this primarily as a conglomerate of oral sex, both male and female. For the purpose of simplicity, I'll be using "giver, receiver, and their" instead of bothering with "he, she, his, hers". And for the sake of readability, I'll try and use short and to the point paragraphs. Now then, being a guy myself, I'll start with guy's receiving.
Just to make this seem a little more official than something written in the wee hours of the morning by a guy in a recliner, I'll go over a bit of the history of fellatio; in Ancient Greece, it was called "playing the flute" and the Kama Sutra (a book I'm sure most committed couples have a copy of stashed away somewhere the kiddies won't find it) has a chapter on it titled Oparishtaka, or "mouth congress". Aside from the religious and social stigma associated with the act, it is generally seen as a dominate act because the giver is most often on their knees, or otherwise in a position below the receiver, unless engaged in the 69 position with the giver on top, on their side or some other weird, and possibly awkward, position. However, and this case is especially true with fellatio, the receiver is placing their penis into an area filled with teeth that could bite, shear, and otherwise maim the tender flesh should the giver be filled with mal-intent (desire to harm, in other words).
Basically, it is dominant because the receiver is usually in a position above the giver, but also submissive because of the serious harm that can come to the receiver if the giver wants. Now then, onto the bulk of reasons I've heard against fellatio, along with tips for the guys at the end.
β’ It's disgusting and dirty: A very sound argument if your man doesn't clean up, but if he does, it's actually rather clean so long as he doesn't blow a fart in your face, which I don't think he'd ever do on account of how easily you could wrap your hands around his nuts and squeeze really hard. Not to mention, that'd be the one thing that would prevent him from ever getting head again. (Forgive the poor attempt at humor; I don't want this turn to into something that sounds like it was ripped from a textbook)
β’ It's so hairy: Nothing ruins the mood, for both parties, than having to stop to pluck pubic hair from your teeth. Nobody likes the feeling of hair in their mouth and knowing it came from their crotch is even worse, especially considering it's some of the coarsest hair on the human body.
β’ It's against my religion: Yes, I actually heard of a girl using this one to weasel her way after giving a blowjob. Now, I'd rather not get into this one on account of how much hate mail I used to get just for associating with LaVeyan Satanism (not the devil worshipping kind, but nobody stops to think about that; they see the word Satan and automatically assume you worship the figure, but this isn't a theological debate), but I will touch on it. From what I remember, she was more than willing to let her boyfriend finger her and go down on her, but a friendly hand job was all he ever got in return, all because she'd been raised Christian. Now, normally, I have nothing wrong with such a reason because it's perfectly reasonably, even if I don't agree with it (I'm a supporter of religious equality and freedom, just so you know), but not the way she used it. If you're a religious person and abhor the thought of going down on a guy, especially a teenaged one, because the Bible, Torah, or Qur'an forbids it, fine, but don't get a guy's hopes up by letting him go down on you! I say this because teenaged guys especially believe that "to give is to get"; in other words, if they go down on you, they're "returning the favor" ahead of time, no pun intended. I'll wrap this one up before I contradict myself by making it too long.
β’ I don't like the taste of semen: Understandable, but not an excuse in my books. For the giver, there're three choices of what to do at the climax: 1) Swallow; it's not as bad as it seems as generally guys tend to thrust up when they orgasm, so the head of their penis is usually towards the back of your throat will only hit the very back of your tongue where there are fewer taste buds. 2) The pornographic "cumshot"; basically letting him ejaculate on your face, chest, stomach, or wherever else. Semen washes off so easily that wiping down with a dry washcloth cleans all or most of it up, so it's not like it is hot lava that's going to incinerate the flesh from your bones. 3) Spit; any guy will tell you that this is their least favorite option; it can also be the worst. If he comes in your mouth, but you don't want to swallow, don't go running off to the bathroom as though you have battery acid in your mouth; not only is this a major mood killer, he'll feel horrible because he'll think he pressured you into it. There is a better way to go about it: if you don't want to swallow, but you don't want him to try and coat your body with his come, let him come in your mouth and then, while staring directly into his eyes, let it dribble out of your mouth onto his dick; I guarantee you that it'll drive him wild! No matter which option you choose, really, so long as you don't do the predictable and run to the bathroom, all are really erotic as guys are very visual beings.
β’ My mouth gets too tired: Alright, I'll give you that one, but think of it another way; if you rest a little to relax your jaw some while sensually stroking him, it'll prolong his orgasm, making it that much more enjoyable on his part and he'll praise you for it, making it a little better for both parties. So, in actuality, your mouth getting tired is a good thing; that's also assuming the blowjob is the main course instead of the opening act. Not all blowjobs have to end in ejaculation, after all.
That covers most of the major reasons I can think of at the moment, but I'm sure there are many, many more to be used. As far as the swallowing of semen goes, while many may not enjoy the taste, there are ways around that, which I'll go into in a moment; first, a medical reason for it directed at the ladies: It has been suggested, and backed by studies, that ingesting semen reduces the chances for preeclampsia, a life threatening disease that occasionally arises in pregnancy. It's been shown that this risk is significantly reduced between couples who frequently engage in oral sex ending in the swallowing of semen and almost non-existent in couples who do so regularly. Now, onto the ways you guys can make the semen taste better for our significant other so that they won't be so reluctant to give head regularly. An all acidic diet, i.e. coffee, cigarettes, and junk food, will make it taste horrible to the pallet, whereas rather sweet tasting things, such as honey, fruits and vegetables, or even a flavored, edible lube coated on either the penis of the receiver or inside the giver's mouth/tongue are able to change the taste significantly, or at least cover up the taste, particularly in the case of someone who likes their junk food and asks for a blowjob at a random time, such as at the movies, driving in the car, etc. Most importantly, don't forget how you feel towards the person as your mind can greatly overcome these things; I once heard from a man who loved Sake (Japanese rice wine for those unfamiliar with it) that "if Sake tastes bad, something's wrong with you". Essentially, it's the same concept; if you really love the person, you may not even notice the taste, whereas if you're just doing it because they asked you and you feel it's your duty, it's going to taste as though someone poured rancid, liquefied cow manure down your throat (gotta love my analogies, don't ya?).