10 Most Inappropriate X-Men Storylines
2. Quicksilver Sleeping With His Sister - Ultimates 3
The almost purposeful grossness surrounding Quicksilver's romantic, and inordinately suggestive, relationship in the Ultimates series is one of the most well-known spots of weirdness in the Marvel universe - although it being common knowledge in no way stops it being profoundly weird.
Bonding over their shared frustrations with their father, Magneto, the pair begin a relationship that initially can be read as platonic if you squint at it, before being revealed to be explicitly romantic in Ultimates 3.
The weirdest part of the whole thing is that this incest is sort of approved of within the comic itself, with Wasp telling Captain America that his horrified reaction is due to his '1944 brain', and not basic common sense.
The Ultimates series is weird enough without this story in it, but it all gets amped up to eleven with this unimaginably sketchy side plot.