It's dangerous. Humans are meant to create strong intimate connections over time. You two will have to fight that off. Very dangerous. I don't recomend it, but I do DO IT too.
I usually leave the girl after the first couple of weeks because they become too attached. With my current fuck buddy I will be more clear with my intentions and reinterate that frequently.
I will say. I only want to have fun. I don't want to hold hands. I don't want to kiss in public. I do want you to feel good by beeing around me, but I don't want to be in a committed relationship. I do enjoy our time together, but I am not looking for anything serious. By serious I mean I am allowed to date other people.
I only DATE other people who are not serious like me. One day our relationship will end, but we will still be friends. We can always trust each other as long as we are open and honest. I care about you, but it's impossible for me to get in a relationship. If you are only intrested in this relationship to change me to get serious with you then I recomend we call it off now, because that is destructive to try to change someone.