This is an articulation of a somewhat backdated thought.
Cutting straight to the chase, when a guy decides he wants to go hang out with someone and it's not gonna be a group outing, he would more often than not rather ask a girl out than another guy.
With that, I will further develop that it is hence okay for a girl to hang out with either a girl or a guy, but it is uncommon for a guy to want to hang out with another guy.
This is because girls have the propensity as well as socially-accepted right to do 'stupid things'. By 'stupid things', I'm generally referring to inane, silly things such as being lame, gossiping, and any other random gay shit that, when done in the company of a fellow male or fellow males, would evoke raised eyebrows and be terribly frowned upon.
That said, it means that when guys go out with one another, there are less things they can be allowed to do. By going on a date with a girl, he acquires the socially-accepted right to act silly and do stupid things with her company.
I believe this scientifically and empirically unsupported theory is further supported by the fact that guys often only ask another guy out when he already has something in mind he wants to do, and the company of another male is largely a by-the-way kinda thing. It is somewhat just queer to for the average joe to ask another dude out just to 'talk', or go chill out. Often, guys interact by doing things, such as going out for drinks or to play pool, and conversation comes about as a by-product of the predominant activity.
I have been in the crappy situation of being with one or two other guys with no aim in mind and we always end up standing in the middle of nowhere trying to decide what to do.
So inherently it is not entirely because all guys are innately skirt-chasers, but because a girl would provide a far better outlet than another guy when it comes to a one-on-one hang out, enabling for potentially more activity with the allowance for doing 'stupid things', and is thus generally a much better hang out choice with regards to gender.
Money doesn't make you happy. I now have $50 million but I was just as happy when I had $48 million.
- Arnold Schwarzenegger
Within Temptation - Memories