It's not like Marvel, up until possibly now, as ever wanted to even broach the subject, right? It's unexplored ground, so you have no foundation to suggest that they woulnd't be up for exploring it.
Like another poster said, it's similar to Marvel not wanting any gay characters in their comics. They just didn't explore it.
I think the sales are an indicator that the whole direction is doing well with the majority so far, not that everyone is ok with the poly thing specifically.
On here, it just seems like it's the same 3-4 people discussing the poly thing as being bad. Everyone else just ignores it or rolls with it.
Seriously. Even just keeping things on Jean, she has a crazy ass life. How can any real person completely relate to what her thought process would be? She's died and resurrected a few times, ascended to actual Godhood, and has telepathy strong enough that when it activated she felt the thoughts of her dying friend. She at least partially forgave Emma back in the Morrison run because of her telepathy, she knew Emma wasn't just some one note trifling bitch, she had genuine feelings for Scott and Jean could feel those on a level none of us could in reality.
And Scott and Logan have so much Ho Yay that they have their own section on TV Tropes. All this subtext has existed for a while, the mutants realized they all love each other and their lives are weird af, why not go "**** it?" and pull the trigger? Lets have some new relationship dynamics with them all acting like adults and with no tedious angst.