Most girls aren't looking for a f*** buddy. If you sleep with them, it's usually doesn't take long for them to want to date. If you say no, most girls, will stop sleeping with you.
If you get a f*** buddy that you don't date....
1.) She's almost certainly banging other guys. Most guys will gladly bang some chick. Girls are more selective. This can lead to awkward situations where you need to visit a clinic because things itch.
2.) Once she finds boyfriend material (and most are still looking for a boyfriend), she'll stop f***ing you.