I think humans have general preference for "reals value" as opposed to faking their reward signals (a.k.a. "wireheading"). Of course sometimes we fake the reward signals, because it is pleasant and we are programmed to seek pleasure; but if we did it without restraints, our survival value would go down. So when someone enjoys "fake values" too much, they will get negative social feedback, because by putting themselves in danger they also decrease their value as an ally.

So a part of mechanism that warns women against "fake alpha males" may be a general negative response against "fake values", not necessarily related to specific real risks of having one-night sex on birth control with a fake alpha versus with a real alpha.

Another part could be this: it is good for a woman to have sex with a man whom other women consider attractive. (If the man is unattractive to other women, perhaps he has some negative trait that you did not notice, so it is better to avoid him anyway, because you don't want to risk your child to inherit a negative trait.) On the level of feelings -- not being a woman I can only guess here -- the information, or just a suspicion, that a man is unattractive to other women, probably makes the man less attractive. (It is a perception bias.) Simply said: "women like men liked by other women"; and they honestly like them, not just pretend that they do.

The idea of a "fake alpha male" (a PUA) probably evokes an image of man who was unattractive to women he met yesterday, and who is unattractive even today in moments where he stops playing by the PUA rules and becomes his old self. Therefore he is an unattractive man, who just uses some kind of reality-distortion technique to appear attractive. -- An analogy would be an ugly woman using hypnosis to convince men that she is a supermodel. The near-mode belief in existence of such women would make many men feel very uncomfortable, and they would consider "speed hypnosis" lessons unethical. (For better analogy, let's assume that this "speed hypnosis" cannot be used to break someone's will, only to alter their perceptions.)