I can't imagine a single reason why the use of any slur, be it based on ethnicity, sexuality, or gender, shouldn't be cause for an immediate, automatic suspension. The only possible justifiable use is in calling another user out, but if Riot were explicitly and irrevocably clear that these words have no place in the most visible esports community in the world, that would be a moot point.
They are already banning based on slurs. Instantaneously banning anyone that used any slur or ratial/sexual/gender-based word would be a gigantic step back in term of security (feeling secure about chatting, not account security).
Banning on occasional (and I do mean occasional) outbursts of slurs would be very bad for a business. The impact of a slur is relative to those present, and in general something will be offensive to someone. Ban outliers outright (people who are offensive to the greatest number of people the most often), warn others, and encourage positive behavior.
If Riot banned someone the first time they called something "gay" and that person was a paying customer, it'd quickly get around that there's no reason to put money into this game if you get banned for the slightest slip in behavior.
Punishment for verbal abuse comes not from a single-issue basis. It's based on how many games the individual is ruining with his mouth relative to how many he is not.
