Do you prefer to spit or swallow? Why? Some people spit after fellating a guy because they dislike the taste or texture of the ejaculate. I've always found that to be a bit odd. I mean, I can understand the instinctive guttural reaction of wanting to spew an unwanted substance from your mouth, but when I look at it from a practical standpoint, it seems pointless to spit.
First of all, it seems almost rude to spit. I've had guys suck me off and then immediately turn and borderline vomit my man goo out. I always just chuckle because I'm not particularly bothered by it. But the visual of someone essentially siphoning a fluid out of you and then turning to spit it out seems funny to me.
The few times I've opted to spit were when the amount of cum was just too much to easily swallow. On these occasions, there also seemed to be a bit of a gross factor going on, so I just didn't want it in my mouth, so out it went... as discretely as possible.
Spitting because it tastes gross or because of the texture, though, isn't entirely logical. Whether you spit or swallow, you will still taste and feel the cum. Whether the gunk slides forward on your tongue or backward makes no difference. So, if you're spitting for that reason, alone, you might as well just go with swallowing.
Some folks are concerned about catching various STIs though oral, and might spit in an effort to avoid contracting chlamydia, gonorrhea, hepatitis A or B (maybe C), herpes, HPV (warts), shigella, syphilis, or HIV. The thing is that once the penis has entered your mouth, you've already been exposed to some of these diseases (herpes and HPV primarily). Once the ejaculate is in your mouth, you have been exposed to any STIs that your partner may have. In most cases, it's while it's in your mouth that you are at the greatest risk of catching any of these. Swallowing doesn't necessarily increase your risk. With regards to HIV, there have been no proven documented cases of someone acquiring HIV through oral sex, but if there is a chance of getting it, it would likely be through an open sore in the mouth rather than somewhere else in your digestive system. Acids in the saliva, throat and stomach kill the virus. You can get some infections in the throat. To be honest, I don't know how far down the throat this is likely to occur. If it's just behind the tongue, spitting or swallowing would likely make zero difference as your mouth fills with cum upon ejaculation. If it's the case that you can acquire, say, gonorrhea farther down the throat, then perhaps spitting might be a more reasonable thing to do. I can't seem to find any information online to indicate if that's a risk.
Of course, one thing you can do to avoid having to decide whether to spit or swallow is to not get the semen in your mouth at all. You can ask your partner to warn you before blowing his load and you can finish him off with a handy. It's not quite as much fun for him to be finished off in this fashion, but you have every right to decide what you're willing to do during any sexual act, and that includes declining to take his fluids in your mouth. Beware, though, that you will still get some fluid in your mouth, regardless. Any pre-cum that comes out may be unnoticed by you and will likely be swallowed during the blowjob. Beware, also, of assholes who insist that you take their load in your mouth. They may not warn you before their orgasm or, worse yet, they'll force their cock deep into your mouth, holding your head down as they cum. If they do this, bite their fucking cock. They're sexually assaulting you. Call them out on that bullshit! Hell, press charges, if you can. If you're into that sort of thing, however, then carry on. Personally, that's one surefire way to make sure I never suck your cock again.
Whether you spit or swallow makes little difference, overall. If you're mostly concerned about being safe, don't let the ejaculate in your mouth at all. Either have him wear a condom, don't suck him off at all, or use the oral pull-out method. Once it's in there, though, just fucking swallow it. With most guys, you can swallow it while still sucking him, and barely even notice that it was ever in your mouth, aside from the taste of it. Hell, you're already swallowing your own slobber as you're going at it, so just add his man juices to the stream of fluid you're taking in.