They're still okay with it and decide to have sex.
Then a few days later they become really attached and possessive. When they find out that I've spent a night with another girl, they get really upset and sometimes even turn against me. I just don't get it.
Most Helpful Girl
Bringing science and biological stuff in here, haha.
Oxytocin is released when two people have sex. This hormone reduces stress as well as increases a "bonded" feeling towards one another. It's kind of nature's way to bond two people so they can raise children.
Also, women are more emotional creatures, while men are more sexual. Because of this, many women look for more meaning or value in sex than men. There are several women that have sex with a guy, then falls for them because of it.
This one is my experience as well as opinion, but I also believe some girls think they can "change" a guy. The girls are most-likely attracted to you emotionally to a degree. If you tell them you are seeing multiple girls unless you say they're exclusive, they'll want to be the "exclusive" girl and have you commit.
Most Helpful Guy
It would be too easy otherwise.
Seriously though, sex is about more than sex for men and women. It's bound up with ego and social status in all primates. A man largely gets gratification from a woman consenting to sex, a woman largely gets it from attaching a man's affections.