As an incubus, Asmodeus felt like he was the most qualified to give an explanation. "Well, when two adults love each other very much OR have ingested copious amounts of alcohol-"
Was it because she had been thrusting her spear continuously without even taking a break for almost ten minutes? I think she was throwing a thrust every second. Perhaps there were some hidden conditions that she managed to reach due to her tireless Undead body being able to do it without any problems like exhaustion or muscle tension or whatever the living weaklings suffer.