What's in it for them? Somebody paying attention to them, free entertainment, boundary pushing with a newbie (always fun), and a free ego boost.
But, what about you? Why do you want to be "friends" with them? Once you learn to set firm professional boundaries, this job will feel much less confusing. Spending time and effort on guys who are not paying you for your time is going to exhaust and frustrate you very quickly. Once you stop chatting/sexting and whatnot, they're either going to leave you alone to get on with your job, or they book and pay. Unless they're very good regulars, your interaction with them ought to stop once they walk out of your door, or it'll drive you bonkers.