why are girls more picky about who they sleep with?

in general men seem to be ok with the concept of sleeping around / having sex with many girls but girls on the other hand are more reluctant to do the same. of course there are social reasons for this. reasons such as the social domination of females by males. if we look at the world right now, we can see female sexuality which changes a lot from one part of the world to another depending on how "evolved" the society is. you would see much more sexually liberated women in countries which rate the highest in gender equality.
but there are also evolutionary reasons for this difference in this behaviour between the genders. the reason is that when a women would mate and get pregnant, it would take her atleast 9 months to 1 year to be able to have an opportunity to make that choice again. the male on the other hand can make that choice of choosing his new mate in a matter of few minutes. so, every time she has sex the female is making a choice which has a much bigger impact on her life compared to the male and hence she would always act more choosy. but this evolution happened in the our long history when we didnt have condoms and other birth prevention tools. given how easy it is to avoid pregnancy now, i think this evolution would roll back and we would see similar lack of pickiness from women as we have seen from men for so long

why do guys like girls with big boobs and ass

the definition of sexy is surprisingly well defined all over the world .. it has been changing off late but still a girl with curves (big boobs small waist) is the most common standard for sexy

there is a hormone called progesterone which is vital for female pregnancy .. helps in maintaining menstrual cycles and supports gestations .. there is another hormone called estradiol which helps in development of sexual organs (supports vagina lining etc) ..  now it has been researched and found out that the the girls with a high ratio of boobs to waist would have as much as 3 times of these hormones than other girls .. 

us humans had obviously figured it long time back by trial and error .. the guys figured out that if he could find a mate with a high ratio of breast to waist, his odds of procreating would go up a lot .. this was learnt by human males over 100s and 1000s of generations and has become a part of the way guys think now without anyone having to tell them .. 

and thats why a guy would get attracted to a "hot girl" with big boobs / ass and small waist :)