You do understand you cannot compare what is portrayed in the game to the actions of other players in the game, right?
Before the ESRB clarified the meaning to 'Users interact', the old language was 'Online interactions not rated by ESRB' which was meant to be interpreted as that the ESRB has no control over what other users of the game will do, since this is not something they can control for when rating a game. The only people who are allowed to control what users are able to do online are the companies that run said games and who maintain the servers. As such, they are allowed to be more strict than what the ratings suggest should they want.
In addition, when you do interact with players in the game, 'Sexual Themes' would be insufficient to describe how the community behaves. Were I someone who was tasked with rating how the community of FFXIV conducts itself in relation to the ESRB rating system, the correct terminology most likely would line up with 'Sexual Content' or 'Strong Sexual Content'.
By the ESRB's own definition, sexual themes are "References to sex or sexuality". Unless things are considerably different in terms of reading standards from when I was in school, you could find things that would classify as such as early as 7th to 8th grade where I am from in the US, and seeing sexuality is in the mix, it could be even lower now based on the curriculum in my area. This indicator should not raise any concerns unless the parent in question is trying to avoid these sorts of topics all together.
The other two content descriptors, 'sexual content' and 'strong sexual content', are a more fitting description of what you will find inside the community. These two are described as "Non-explicit depictions of sexual behavior" in the case of sexual content and "Explicit and/or frequent depictions of sexual behavior" for strong sexual content. The more details that are given, the higher the rating as it becomes more explicit.
From my schooling, I do remember books that would fall into the 'Sexual Content' range that I would've had to read when I was in 10th - 12th grade. Books containing something like this would be few and far between, and when they did contain something like this, it was rather tame compared to what you would expect from the community as nothing can be explicitly said at all. As for strong sexual content, while I cannot speak to current schooling standards, books like this were not a part of the curriculum while I was in school. An example of a book containing something like this would be 50 Shade of Grey, which meets the criteria due to the contents of the book.
As such, your argument is faulty since it assumes what the players do is within the bounds of the ESRB rating, which is false. It is possible to encounter behaviors that escalate above the rating indicated on the box from the players.
While you are not responsible for the failings of parents, you are directly responsible for conducting yourself in a way that is suitable for individuals who can be, at minimum, age 13 as per the user agreement you signed when you started playing this game:
Whether or not the GMs want to crack down on the behavior is a topic for another day.3.3 Profanity and Offensive Language. You may not use profanity or any language that a reasonable person would find offensive. The Game is for players aged 13 and older. You agree to behave accordingly.
Rules and Policies - Final Fantasy XIV User Agreement section 3.3
The only question I have for you is why are you so adamant when it comes to this particular discussion? If I am to take you for your word, which I am willing to do, it is clear you do not like the behavior of a fair chunk of members inside of the community. However, you seem opposed to having guardrails regarding the community's behavior as well. As such, what is your main concern when it comes to resolving this issue if you are concerned with resolving it in the first place?