If you Google "homophobic slur" the AI Overview comes up with the following: "A homophobic slur is a derogatory, insulting word or phrase used to degrade, marginalize, or express hatred toward individuals based on their real or perceived sexual orientation, typically targeting gay, lesbian, or bisexual people."
If we use that definition we can ask two questions:
· Is "gay boy" a derogatory, insulting word or phrase?
· Did the footballer use the phrase to degrade, marginalize, or express hatred towards to gay people?
There are three meanings of "gay" we can consider:
· Cheerful and carefree. (Positive)
· Homosexual. (Neutral)
· A slang word meaning something like "definitely not cool". (Negative)
I think we can safely say that the first definition was not in the footballer's mind.
As to the second, possibly. What reason would the footballer have had though to literally suggest in the middle of a football match that another footballer was gay? It is of course not quite that simple. The footballer may entertain the idea that being is on a par with being a cheat, liar or fraud. If you say someone is a cheat, liar or fraud that is a slur. Asserting someone is gay ought not to be a slur if you consider there is nothing wrong with being gay, but what if you consider calling some is gay to be an insult? These days any sensible famous straight person called gay will shrug it off asserting s/he does not have a problem with being called gay. If they sued for libel they would be taken to task by the gay community.
As to the third, see here
https://en.wikipedia.org/wiki/Gay#Generalized_pejorative_use which indicates a lack of agreement on how the pejorative use should be regarded.
Only those who know the footballer will have any idea what his intention was. The fact that those imposing the penalty considered he did not wish to be intentionally homophobic suggests they considered the third definition came into play. I do though have a bit of a problem with the concept of "unintentional homophobia". It strikes me as approaching a contradiction in terms like "unintentional violence". In any event, they seem to have accepted that there was no intention to degrade, marginalize, or express hatred towards gay people, which, if the definition quoted above is accepted, means there was no homophobic slur.