Emotional and psychological complications, sometimes long-lasting. Early loss-of-virginity age, which many people still care about in a marriage partner.
Empirically, people who lose their virginity early (< 18) tend to be hypersexual as adults.
Empirically, people who lose their virginity early (< 18) tend to be hypersexual as adults.
Seriously though, any "emotional and psychological complications", etc. are only because of our societies ridiculous, puritanical, anti-sex attitudes. It's really way past time for society to move past these dark ages attitudes about sex. Sex is natural, and even better, it's FUN! :-)
I don't have any problem with an isolated casual sexual encounter. As far as I'm concerned, that's none of my business. However, I absolutely hate living in a culture that glorifies it, to the detriment of loving relationships. I also hate living in a world where the vast majority of available women have such awful attitudes as to be unmarriageable.
Those "puritannical attitudes" exist for a reason. Some people take them way too far, and their application is very unjust (e.g. gendered double standards); but they shouldn't be cast aside as relics of "the dark ages". There are many great things about loving, long-term, monogamous relationships and family formation.
Men probably have just as many shitty attitudes, but I don't encounter them directly in my dating life (only indirectly, because most women I know have been with some awful men).
The other half is that a person with a large number of prior sexual partners is more likely to cheat in a long-term relationship, less likely to be sexually satisifiable by one person, and less able to form the intense bonds of trust and love that a relationship requires.
Thus, the "puritannical" stance on casual sex is as relevant and useful today as it ever was. In fact, it's more useful now than it was 1000 years ago; although STDs are much more curable, raising children is considered to be a 20+ year, shared process, making the intact family desirable.
What's disgusting is the double standard-- the stud/slut dichotomy-- that allows men to behave badly with few consequences. My opinion is that women have the power to end the double standard; if women stop being attracted to promiscuous men, for a man to be slutty will be seen as being as foolish and self-damaging as it is for a woman, and the double standard will disappear.