why today it is too hard to find friendship between men and women?

Men and women were not really meant to be friends,but true friendship just happens,regardless of the gender,but I think the best male friend a woman can have,is a gay guy,seen it all the time,but a regular male could be just as good,but I've seen more gay guys as women friends though!

Several reasons.

1. There is no "just friends" DNA code. The majority of the attraction and mating process is done "subconsciously" by your body.
Human beings naturally produce hormones that constantly push them to mate. Not be "just friends." If a straight man and straight woman spend time together then their "hind" brain thinks they have consciously chosen someone to mate with so will start trying to get some pregnancy happening.

2. Socialization, social training and indoctrination, doesn't really promote it. Name a t.v. show where people who were "friends" didn't end up dating or banging each other.
Name some "just friends" web sites where no one dates and people successfully find their "soul friend."
You ever been on a "just friends" cruise?
You ever see a "just friends" night at the bar where "friends" drink free?
If something doesn't receive social validation, then it isn't being reinforced, and it loses importance and desirability.

3. Communication. Men and women communicate differently.
If there are easier alternatives providing the same benefit, then people are going to prefer them.
In today's world communication is becoming ever more specialized, individualized.
You use a different manner of speaking at school, in your work, with your family, and with your friends.
Why spend time trying to develop a relationship that requires 10 times more work in communication when it's just easier to be friends with someone of the same sex.

4. The push for "equality." Mostly this has been promoted through "everyone is the same." If everyone is the same then it means everyone is competition. And if no one provides desired or helpful skills, then there is no purpose to a "friendship."
People are going to seek out those that can be utilized for something they can't do themselves, not for social unity.

5. Isolationism and control. People like to have control over their environments. Look at urban sprawl, houses in the suburbs. Front yards are all the same, private backyards are all different.
Look at facebook and cell phone culture. Keeping people at a distance and controlling what information they have access to and when.
Wanting constant control is not conducive to relationships. Friends or otherwise.

That's probably long enough.
There's ton more reasons.

friendship can exist between a man and a woman.
I have friends who are guys. it is not impossible.

I agree I have female friends I have known my whole life.