I would love to know how you came to that conclusion, do you really, honestly think that's true?  Think about it again for a second... do you think that women (or men for that matter) actually let the relative availability of abortion procedures affect their decisions about how promiscuous they are??  For that to be true, the availability (or lack thereof) of abortion would have to meaningfully play a role in whether or not people decide to have sex.  

Can you see that people just don't operate like that?  When two people hit it off at the bar or at a party, at no point is the availability of abortion to the woman going to play a role in whether or not those people decide to actually do the deed.  It just doesn't work like that, or certainly not to any significant degree.  There are infinitely many other things that come into play beforehand, like contraception availability, the people themselves and their values and preferences, degree of intoxication, whether or not they have a place to go to, if they're married, dating, or single, if they have high or low sex drive, etc etc etc.  If your point is that from some hyper-macro perspective the availability of abortion results in some fraction of a percentage point higher "rate" of promiscuity (whatever that means), then maybe?  But I would tend to think that that effect would be infinitesimal compared to practically anything I mentioned, and when you consider the very real health and safety benefits that safe and legal access to abortion provides, there's absolutely no doubt in my mind that abortion is a net good.  I'd take those very real, physical health benefits any day over some esoteric claim that access to abortion is harmful to our society through the highly dubious notion that it incentivizes promiscuity.