Well there's a whole pile of ways. If you are a girl, and other girls find out, you could easily be labelled a slut which obviously has a negative effect on your self esteem.
It could also make you more emotionally attached to whoever you slept with, which brings on feelings of love etc. Then if the person leaves you it brings on pain, clearly.
As for guys, I'm not one so I don't really know, but I'd imagine maybe feelings of guilt if you weren't ready, or feelings of being awesome if you were cuz that's how guys seem to think.