This one came out of left field-- or should we say out of the movies, since the very idea didn't even seem plausible until Professor X and Mystique shared the screen together? After Professor X died, the X-Men discovered that not only
had he left the school to his wife, but that his wife was Mystique herself.
There is absolutely no mention of this prior to the reading of his will, making it clear that his students were baffled (or, in the cause of the callous Snow Queen, delighted) by the knowledge.
It's fair to guess that the two slept together in their marriage, although it's obviously anything but your standard case of matrimony. Most people speculate that this was simply a way to tie in the movies, just as Raven's bumpy, naked blue skin in later comics did.