Is this new or has it always been there and we just didn't see it?
In United Airlines' Contract of Carriage, we found this language under the "refusal to transport" section (meaning they won't let you fly if....)
REFUSAL TO TRANSPORT UA RULE: 0035
UNABLE TO FIT INTO A SINGLE SEAT IN THE TICKETED CABIN;
-UNABLE TO PROPERLY BUCKLE THE SEATBELT USING A SINGLE
SEATBELT EXTENDER; AND/OR
-UNABLE TO PUT THE SEAT’S ARMRESTS DOWN WHEN SEATED
So in other words, if you're too big to fit in a single seat, and you refuse to upgrade to a roomier business or first class seat (assuming one is available), then they won't let you fly.
You can also be refused if you're barefoot or have a "malodorous condition"--but nothing yet about whether you're giving someone a smooch.