First of all, lets make it clear that it is honestly your and your gfs situation. Dont lie, you can be easily seen through.
Secondly, to the point: this is the fvcking perfect chance for you to go salsa with her or somebody else. Dont you understand that salsa is one of the most sexy and romantic dance ever? No, you do understand; you are just fvcking insecure. What the fvck is with the "I dont like salsa." That is fvcking bullshvt. There is no fvcking one person in the world who is confident and doesn't like salsa. Either you are fvcking insecure with your salsa body or you dont know how to dance. And if you dont know how to dance, then learn to dance. And if your body aint sexy then get a sexy body. And if you dont want to learn to dance then let your gf dance with other guys. What is the problem here? You are the only problem. There is no problem with anything else.
Even if she ends up leaving you, it is your problem. Dont go blaming her, dont go blaming others. Only the weaks blames others. You might not have been interesting enough to keep her attraction. Still it is your problem. Excitement and romantic are very close alike. If she is not excited by you, she will sooner or later lose romantic feelings also.
"but if your gf wants to, in this case, join salsa lessons and go out to dance salsa...what can you do if you do not like the idea? you can tell her you do not like it and then let her decide if she wants to do it anyway.
I dont know what else you can do besides telling her you disaprove it."
Oh my fvcking god. You've been her ever since 2011 and you are still that lost? Why the fvck dont you like the idea? The only reason is that you are insecure. You are afraid of her cheating on you. And why the fvck are you afraid? Because you have no other options. And you are insecure. Deep down, you are already seeing and fearing that she could cheat on you any moment. Otherwise why bother? Why dont you like the idea? If you are confident that she wont cheat on you, there aint no reason to dislike the idea. On the contrary, (I am repeating this so many times that it goes deep down into your brain) you should GO dance with her yourself. As you can see, you seem to be pretty low in society hierarcy. I dont even think you deserve this girl with that much insecurity.
If she is honestly attracted to you, she will not cheat no matter what. And if she wants to cheat, she will find a way no matter what boundaries you set. On the contrary, it is for the very reason that you set boundaries and end up a fvcking insecure controlling boyfriend that she loses attraction.
As ive said before. Let her dance salsa, and go dance with her or cry with your insecurity and prepare for getting dumped sooner or later.