During sex, or even just in a domination/submission relationship, spitting can take place and some find it extremely erotic. Spitting can be addressed and seen in two very different, as either degradation or acceptance. Both of the experiences are very contrasting, and so you need to make sure you know exactly which way you want it to happen.
The first is that of domination/submission, where the receiver is being punished for something and is spat on to show they are not worthy, allowing the dominant to establish themself as a judge of this person. In this situation it may be accompanied by harsh words and insults to reinforce the spitting action, and they are degraded by the dominant figure. The second way is where the receiver of the spit does so to show that they accept everything about the person spitting on them, and in this situation many will choose to take their partner's spit into their mouth and swallow it. They are saying that they love everything about them and accept them as they are, which is very different to the BDSM spitting.
If spitting is your thing then you need to discuss with your partner what type of spitting you want, as they are very different. Whichever context, you will need to establish who will be spitting and who will receive the spit. If you wish to do the spitting then ensure your partner is comfortable taking your spit and if you wish to be spat on, check that they are happy to do this. While it may sound completely acceptable to you, for some it is an unusual request and it is best to establish some ground rules beforehand.
When getting down to it, it is worth checking again that you are good to go. If everything is still okay then just enjoy the moment. If not, do not try and force your partner. It could make things more difficult for you both in the future. Over time they may still come around to the idea, so don't lose hope!