heartbroken - I think it's because men are guilty of the same thing too.
We overinvest, we meet someone online and start romanticising about them, about how great we would be together. We form that picture in our mind of who we think they are, and then whatever they say online we fit into that image. And we start to get attached to that image, and we tell it what we think about how great we'll be, because we believe it.
Then we meet and we have to do some mental readjustment, but if it's not too much readjustment then we go for more dates. And we sleep with them, and sometimes we are happy because the person replaces the image, and sometimes the person does something weirdly out of character for who we think they are, and we go off them. This is compounded by the fact that women get more attached after sex, and let barriers come down, and suddenly they're even more different to who we thought they were.
Some men, of course, just pretend in order to get a shag, but I think the majority just can't reconcile the reality with the ideal, and so we distance ourselves from the real person in order to go on and find someone else who fits an ideal more perfectly. Because there are so many more profiles online. Chalk this one up to experience and move on.