I lose all desire for a girl once I've slept with them
Why do you lose desire after you sleep with a girl?
I just get bored and rapidly lose interest
It is understandable to lose interest in someone after being with them awhile. But you should not take advantage of them if it was only for sex.
I do it because I feel good knowing that I just used her.
Feeling good at the expense of others is wrong. How would you feel if I treated your sister that way?