What exactly do you mean by "nice" anyway? Like an honest, self-respecting woman? One that isn't sleeping with a different guy every night? Because those aren't mutually exclusive. Gonna go out on a limb and say that most women want some sort of relationship with a boyfriend, not just a fcuk friend. Women seem to run a bigger risk of getting attached to people they sleep with, so they tend to cut their losses by not engaging in relationships where the guy's only down for sex. Men seem a lot more capable of separating a committed, loving relationship from a basic sexual conquest.
My boyfriend made a good point, an open relationship is the ideal in between, but he said the terms have to be very clear-cut. In an OR both people spend the majority of their time together, but hold no hard feelings if the person chooses to fly solo every now and then. Most people aren't capable of this kind of arrangement. FWB tends to encompass two people having sex for fun, but outside of that they don't really spend time together. If/when they do spend additional time together, typically, one person starts to get attached and wants more than just the physical satisfaction. Either way, in FWB or OR, someone tends to get hurt which is why a lot of people don't even bother.