X-Men #6: "He's just a man in swimming trunks!"

I'd known it was inevitable, but I'd purposely avoided finding out when X-Men's first guest appearance was. Here we have it - in issue #6, Namor, the Sub-Mariner, appears. Curiously, although the full Brotherhood line-up recur, Quicksilver and Scarlet Witch are singled out on the cover.

Namor is one of the original Timely characters from 1939, and was reintroduced in Fantastic Four #4, a couple of years before this. Here he's given enough of an explanation that readers who hadn't read that can get the jist - with editorial footnotes referring to Fantastic Four #27 and Avengers #3.
Magneto and Xavier realise that he must be a mutant at about the same time, and try and recruit him. Naturally, their teams collide, and equally naturally, Namor's libido wins. In a shocking development, Namor refuses to ally with either side. They are, after all, surface dwellers. A few hundred posts from now, we'll see how he'll eventually join the X-Men, alongside Magneto!

Wanda and Pietro briefly break free from Magneto's control (or rather, Pietro is captured and Wanda refuses to abandon him), but return to him by the end. The last panel provides a slashy moment, as Jean proclaims, perhaps little too hard, "I'm glad to see them go! That Witch is much too attractive!" Course you are, Jean.

Continuity notes

Introduction of Namor into X-Men. Magneto uses his powers to produce an astral projection to search for him (as does Xavier). The phrase "Xavier's School for Gifted Youngsters" appears.