Why do women get jealous when their man has a female friend?

My ex had a childhood (female) friend who began calling around to see him at his workplace and having a morning coffee with him! He insisted nothing was going on - despite her putting the hard word on him years earlier! We were married then as well. I knew if I made a big deal about it, he would continue doing it just to torment me. She did it because, with me being overweight, she thought she was a 'better catch' and could have who she wanted. lol - And yet, after I left him - he turned to another woman without even considering this one! How smug was she feeling then? I KNOW if I had done it with a male friend, he would have gone ballistic. He was a lot more jealous and possessive than me!   Bottom line, if you REALLY care about your partner, you wouldn't put them through this!

Females in the animal kingdom rarely rely on the male for survival.  They can generally handle that themselves.  They just want the sperm from the best male, which is a virtually unlimited supply.  1 male can often impregnate hundreds of females.  So there's no need for jealousy.  As long as the individual female gets what she wants -- pregnant -- it makes no difference to her who else he's banging.  Plus there's safety in numbers.

Whereas human women have evolved needing a man to help her to gather provisions, since women always spent such a large portion of their adult lives pregnant or nursing.  And there's only so much of the resources he gathers to go around.  So jealousy is a reaction to a perceived threat to her resource supply.

I suspect this is why women seem quite happy to be part of an unofficial harem around extremely attractive men.  They aren't trying to get a long term relationship out of those men -- they just want sex.  And they want it because their instincts tell them that those men are the best to breed with.  So as long as they're getting semi-regular sex from those men, they don't really get jealous.  Or at least not to anywhere near the degree that men do.  Because those guys are for breeding, and you just need 5 mins every now and then to make that happen.

I could go on, but I'm tired and I'm sure you guys can extrapolate where I was going with that.

Men don't have any use for women they aren't interested in having sex with. Women who aren't naive realize he's lying.

I got stuck in the middle of a marriage problem as a married woman I know of who wanted to be friends with me. I thought she wanted to have an affair at first or was very lonely. We went out a dozen times and I stopped it as she seemed way to interested in me for a married woman. Her husband was okay with her meeting up with me. But a few tongue kisses and one weird groping and way to many texts. She tried to explain her behavior as her crazy personality and no big deal. 

They see them as a potential threat to the relationship they are in with the man.   If she has a falling out with her man, then the man will probably seek comfort with the female friend.

Fear of resources transferring to another female. 100% or 10%, doesn't matter.

That's not true, Women can be jealous too if they see their men have female friends and you shouldnt say something bad about men 

Because she might be more than a friend, obviously.