Semen spilled because of your over arousal. Too much "heat" and too much desire in your mind. You can't accidentally practice black tantra because it requires the will to do it. Either way, a single act of any type will not doom or save you. There is no need to worry about it causing your damnation.
Nevertheless, you must be careful with your use of sexual energy. Kissing should be used to show affection, or for the couple to prepare for the sexual act. I would not arouse the body intentionally (by kissing or otherwise) without going into the sexual act. If you are not ready, then you should probably stop making out. It is too dangerous. Likewise, do not enter in sexual relations with anyone until you are as sure as you can possibly be, because the sexual act forms a strong karmic bond and it is not something to take lightly.