It is true that male and female friends ships in adulthood are extremely hard. If you are young, opposite gender friendships are common and usually everyone is involved with everyone.
Eventually though you grow up, get married and have kids. Now, if you introduce your guy friend or girl friend to your spouse immediately from the begging and agree on boundaries it can work out fine, no drama. Especially if they are also in similar situations.
I don't really have friends that are girls nowadays but it is a fact that my wife wouldn't feel comfortable with me starting friendships with girls and hanging out with them on my own and I respect that greatly. Because I also wouldn't feel comfortable with her hanging out with guys on her own. We have been together 20 years.
We usually hang out together with any friends we have (though that includes our young kids) Obviously if I am invited to a dinner or a night out organised by my co-workers I will go by myself but that is in a setting of multiple coworkers not just me with a girl.
So I don't necessarily believe opposite sex friendships are impossible but when you are an adult, you need to ensure that certain criteria and boundaries are fulfilled and depending on the situation this can be really hard. Otherwise things can fall apart quickly.