It's the distinction people make with things done while drunk. Sexual arousal markedly reduces a persons inhibitions. Because people see that state of lowered inhibitions as "not themselves", once they're no longer aroused the various anxieties and pressures they function under normally are re-established they see retracting their decision as OK because it's what they would have done "if they were being themselves." It's morally unjustifiable IMO, and smacks of emotional immaturity, but that's where most people are. 'Just because I did it doesn't mean it's something I would do!'