Anon asks: "Why is it so easy for some people to have casual sex and not get feelings afterwards whereas others are very affected by it"
Some people can have sex with out strings attached and some just can't. Its all just about preference. Usually people who have sex casually, have sex with other people who are with in the same mind set. I still think its just as important to let it be known from the beggining that this type of behavior is either tollerated or not. In other words if you are the type of person to have casual sex with out any sort of emotional feeling behind it, You should let your partner know.
There is NOTHING wrong with casual sex as long as no body is getting hurt and you are being protected. Just like there is nothing wrong with having meaningful sex with someone you care about. Neither one of these are a bad thing, It just depends on the person. It doesn't make you a slut or a whore, it just makes you someone who likes to casually have sex.