All statistics about number of sexual encounters are based on questionairres. Men tend to exaggerate by estimating, women tend to understate due to attempting to count the numbers which results in forgetting a few here and there.
There is actually research about that difference in perception.

That being said, promiscuous people have been linked again and again to...
... mental instability and higher rates of mental illness
... higher rates of STDs (It's a numbers game)
... be more likely to be a single parent (numbers game)
... have higher rates of abortion (numbers game)
... have less stable relationships and higher divorce rates
... have increased chances of having been in an abusive relationship
... be more likely to cheat
... be more likely to become addicted
... be more likely to come out of a broken household.


Want some sources?

http://cdn.freedomainradio.com/FDR_2899_Marriage_Partners_Study.pdf
-> Page 19-25. Especially Page 24-25 is quite relevant.


http://www.everydayhealth.com/longevity/can-promiscuity-threaten-longevity.aspx

https://medicalxpress.com/news/2013-02-multiple-sex-partners-linked-drug.html

http://news-releases.uiowa.edu/2011/june/061411paik_study.html

http://www.barbararisman.com/uploads/7/8/9/2/7892654/allison_and_risman_social_science_research_final_paper.pdf

https://www.ideals.illinois.edu/bitstream/handle/2142/42210/Saroj_Hardit.pdf?sequence=1
-> Promiscuity is linked to sexual aggression.
-> Page 59. "The number of sexual partners was positively related to parental violence."
-> Page 70. "As the number of sexual partners increased, so did the propensity to sexually aggression."


http://www.bradley.edu/dotAsset/165862.pdf
-> Page 4. "Perhaps the strongest personality correlate of risky sexual behaviour is impulsive sensation-seeking"
-> Page 8. Promiscuous people are more likely to be impulsive, have low agreeableness, low conscientiousness, sensation-seeking and a higher level of sexual infidelity.
-> Page 14. "People who describe themselves as more unfaithful tend to have personality traits linked to a lack of trust and empathy (i. e. low agreeableness)"


www.sociopathicstyle.com/psychopathic-traits/
-> Point 11. Promiscuity is a common symptom of psychopathy. Goes hand in hand with the other study mentioning impulsiveness as it shows it here on point 14.


http://ink.library.smu.edu.sg/cgi/viewcontent.cgi?article=2215&context=soss_research
-> Page 8. Higher amount of sexual partners is directly linked to higher degrees of psychopathy, narcissism, machiavellianism and an increase of any combination of those 3.


https://genepi.qimr.edu.au/contents/p/staff/CV262Bailey_UQ_Copy.pdf
-> Page 4, Table 1. (Note: Sociosexuality = Parameter for willingness to engage in sexual activity outside of comitted relationships)



To put it into my own words: I think promiscuity is a symptom of mental instability and is a way of self-destructive behaviour, just like i. e. Obesity (not physical-health induced obesity). It's a huge red flag objectively, biologically and evolutionary for anyone who wants a comitted and successful relationship.
Thus there is no logical argument to me that promiscuity is in any way to be seen as positive