She doesn't have fun at parties (any parties, ever) She isn't having fun now (at this party, now) You may be getting confused because of the possessive have, which is a static verb: ‘I have two children’ [never ‘am having’] ‘I have an idea’ [never ‘am having’] So when it comes to possession, have is always used in simple tenses. But in all other situations you need to use progressive tenses when talKing about the present moment: I am having my morning coffee now. She can't talk to you now because she is having a shower. We are having fun.

Isn't having any fun ... because it is happening right now.

When you're talking about something that is happening now, you use the present progressive. Key term in this sentence is "right now". The present simple is for something you do a lot, or something that is always true. "She certainly doesn't have any fun" means she never has fun. It's not just about now.