What's wrong with having sex with a lot of people?

If a person is not religious, doesn’t adhere to cultural expectations surrounding sex, is safe (condoms, birth control, etc.) why is it a problem to have sex with a lot of people?

I’m not talking about nymphomania (if that exists), but just having sex without having to have an emotional attachment.

The word slut isn’t necessarily bad, is it? Why is it a big deal?

If you only live one life and love sex, why is it a problem for other people? Why judge?