Evidently Alia likes it that way.
And X liked it in X8 when Alia got bustier. At least now when the sexual harassment comes up, he'd had gotten a good grope in first.
Why else do you think the majority of their interacting was via Comm Link? And Alia was so nervous when X was physically around her?
Also, more evidence. As said, in ZX, X merges with Aile and Zero merges with Giro. Zero chose to merge with A GUY. X got a hot chick who turned into an even hotter chick in ZXA. And when Zero became part of the equation, it was because Aile would die otherwise. So, X deals with it, but remained the dominant Biometal in all transformations, even with Omega. How? Because they agreed that she was too hot and have you looked at OX? Skimpiest outfit of them all. Did you think that was just an accident?
At least Model F later got a girl of his own. Atlas is another hot piece of ass. And Model A got Ashe.
But, then again, Axl and Fefnir were pervs from the beginning. X has been scheming for centuries. Who knows how many other women were groped and harassed by him? He saved the world eight times, EIGHT TIMES. With that much respect, you can get away with a lot.