If it's in Morsy's contract then it's a different matter and not what you were arguing.
It is precisely what I'm arguing as there may well be very strict contractual requirements (e.g. on clothing), but also other contractual clauses around conduct and acting in a manner that aligns with the club's values etc.
So my understanding is that there are very strict contractual requirements on clothing, which cover all items of kit with the exception of items considered to require personal fit, specifically boots and goalkeepers gloves. So a player is contractually required to wear the shirt, shorts, training kit etc (with logos etc etc) that the club determines. It may be that the issue of the armband is a weird loophole that isn't covered - I've no idea. But I imagine were the club to determine that the shirt has a discrete rainbow logo on it then players would be contractually obliged to wear it. In fact there have been instances where players have refused to play in shirts with betting logos on - the result wasn't that they were able to wear an alternative shirt, but that they did not play.
As to support for any minority, leaving aside your attempt at the ad populum, it's very obviously a political statement, and that's why offset made. If Morsy wore an armband of his own supporting those in Gaza, then he would be breaking the rules of it being political.
You and I can argue whether the rainbow is political and we clearly disagree. But our view isn't really relevant here is it. The FA does not permit political or religious messages/slogans to be worn on football kits. The FA has clearly authorised both the rainbow armband and poppies on shirts as acceptable, so does not see them as 'political' or 'religious' under their rules.
So as far as I understand it, the rainbow armband was allowed by the FA but not obligated, hence why the FA decided that the Morsy situation was a matter for the club to determine - in effect whether refusing to wear it broke their rules or his contract. It would appear that they came to a compromise, but a different compromise was entirely possible (a principled decision by Morsy to step aside as captain during the period where the club wanted to show their support for the LGBT community through the wearing of the rainbow armband).
The Guehi case is different - the FA clearly authorised the wearing of the armband, and presumably Crystal Palace wear not prepared to accept the same compromise as for Morsy. However the message that Guehi wrote on the armband was deemed to be a breech of the FA's rules on religious messaging and he has been warned although the FA have chosen not to take any further action at this stage.