The incident doesn't have to have taken long. If he was already aroused and ready to go, so to speak, he could have come over, spooned her, lifted her skirt, and ejaculated, all before she became fully awake and aware of what was going on, overcame her disbelief, and responded. He also might have ejaculated before the incident, and just wanted to leave the ejaculate on her for his own reasons.

I didn't really get what was funny about it. Either it happened, and the man is little better than a rapist, or it didn't, and the woman has made an accusation that will stay with the poor guy for the rest of his life.