There are two disadvantages I can think of off the top of my head - one, alluded to by others on this thread, is that the sex act is inherently dangerous in terms of disease communicability, the more so among primitive peoples with limited medical capabilities. As long as these are isolated from other family groups the problem is minimized, but as soon as there is interbreeding between groups (lack of which has its own genetic disadvantages) we get an immediate disease gradient. Promiscuity kills - anyone who has followed the dynamics of the AIDS epidemic (or any other venereal disease arc) knows that the math involved dictates that multiple partners makes risk increase exponentially.
The other disadvantage might be described as a political one in terms of paternity - for better or worse, succession of leadership within family groups governed by a patriachical structure tends to depend on paternity, within matriarchical structured groups somewhat less so. One might think that within the latter promiscuity might allow a survival advantage for the group as a whole, and perhaps it does, but observation tells us that the patriarchical form seems to be the more successful survival strategy, due possibly to the dynamics of aggression between family groups. One might take the history of the Zulu nation as an example: originally matriarchical, its accession to nationhood resulted from, or in, depending on one's point of view, the changeover to a patriarchical structure.
Why identity of paternity seems more important in patriarchical societies (if indeed it is; I might be entirely wrong in this hypothesis) may be linked to differences in the psychology of mortality between the two sexes: a woman knows she has reproduced, a man must always doubt in the absence of some other form of societally-sanctioned reassurance.
Thrown out for consideration - if, on the microscopic chance that I might be wrong about this stuff (snicker!) I'm sure my fellow FReepers will let me know in their usual diplomatic fashion...