More like 2 times. Still, it's an inter-generational (non?) relationship between two consenting adults. Difficult, yes, and possibly subject to public tsk-tski-ing, but not outside the realm of possibility. Xavier is hardly one to take into consideration public sentiment, in how he carries out his personal affairs.
What i'm more curious about is the extent to which Jean recognizes the nature of Xavier's affaction -- not only is she more intuitive (by personality) than Scott (who is naturally very stoic), but she and the Professor work in such close contact, as telepaths. Surely, she must have picked up stray thoughts confirming his feelings, every now and then, beyond the more ambiguous social/behavioral signals.
And, perhaps more crucial to the immediate storyline -- does Xavier have knowledge of the Professor's real feelings?