For exactly the reasons you said. She couldn't help losing herself in that moment. That unrestrained lustful passion that you heard through the walls. She is going to remember how incredibly sexy that felt. How sexy she felt. And now she knows just how easy it would be to feel that way again.
And it might have taken you twenty years to get there. But now that you crossed that line once. So much easier to cross it again. Especially now that you know how intensely intoxicating it was to hear her lust.
All it will take is that right spark. Her getting a little drunk. A guy's hungry eyes lingering on her body. You daring to remind her of that night. Now you have both had a taste, only a matter of time. When, not if.