I don’t know the app, but if they’re literally harassing you, this might also just be someone with two accounts talking with themselves.

If they’re different people… “Friends” is just a label. It means “people being friendly with each other, helping each other out in a mutually beneficial relationship, not sexually involved”. The “not sexually involved” part is usually implied because if they were sexually involved, you’d call them something else, “partners” or “fuck buddy” or similar. But if someone is sexually open, not doing the standard “partnership/relationship” thing often or at all, then they might just drop the additional description of “friend” to have the property “not sexually involved”. It is literally called “friend with benefits” often. So yes, “friends”, however someone defines that label, may include talking like this.

What you’re really asking is “in usual society, do friends usually talk like this?”, to which the answer is no. But you’re also implying that friends talking like this, or someone having a slightly different definition of “friend”, is somehow a bad thing (“gross”). Which it is not, in a consensual relationship, anything goes. The bad thing is the “harassing you” part. There is no need for you to socially shame their sexual behavior by calling it publicly “gross”, the “harassing you” part is already shameful enough. I would focus on your boundaries and enforcing them, you are fully in your right to do that, and not focus on their behavior with other people.