The first sex scene doesn't actually triggered by the blowjob-behind-their-back scene. Its just an unfortunate coincidence that the required level of lewdness and/or lust is achieved by the latter scene which in turn triggers the unavoidable former one. So what you have to do is avoid any suspicion raising activities (and lower it below 25) before the first sex scene because that gives a +75 suspicion and increase lewdness by safe non-suspicion raising methods.
You need to have huge level of Lewdness as well (and keep her Lust at max just to be sure).
Must be a glitch because she can be called indefinitely afterwards. Or perhaps its the idefinitely part is the glitch