Is it safe for my girlfriend to swallow my cum? Should I just do it, or ask her before I do it?
After polling the heterosexual and bisexual women I know who like to suck cock, there seems to be a majority opinion that nice guys ask first. Some women are happy to suck dick, but they don't like the way cum tastes. Other prefer to swallow only if they have some kind of emotional relationship or connection with the man in question. (I also want to mention that there are a few women who expressed great enthusiasm for the taste of cum. One of them said, "If you don't get a hot shot in the mouth, what's the point in taking a dick down your throat in the first place?") But there's no way for you to know which category your girlfriend fits into unless you ask.
I should also point out that there are some infections that can be sexually transmitted in cum. HIV is the best-known among these, but cocksucking can also carry the risk of transmitting gonorrhea, syphilis, and hepatitis. So most girls will appreciate the offer to use a condom. This does cut down on sensation unless you've done your safe-sex homework and jacked off with condoms so you're used to how it feels to move it up and down your shaft. A dab of lube in the head of the condom can make it feel better.
If a man is healthy, however, there's nothing dangerous about swallowing cum. Folklore would have it that avoiding asparagus, coffee, and cabbage will make your cum taste a hell of a lot better. So while you're waiting for your girlfriend to make up her mind about tasting your cum, you have some experimenting to do to see if you can make your ejaculate aesthetically more pleasing.