Unfortunately, society teaches women from a very young age that they must compete with each other for the attention of men. That makes it very hard to form true, lasting, supportive relationships - especially for those girls who take the "getting the attention of men is more important than anything else!" message to heart, the ones who are most sexualized. Unfortunately, they are also most likely to be dissatisfied with the attention of one man, and are often willing to leave a guy if they think they have a chance with a male of higher status in any sense of the word - or to cheat in general.
If you want a healthy relationship, I'd recommend holding out for a girl who has decent relationships with other females as well as having male friends.