Unlike many people have told you, it's perfectly natural to feel that way about a girl/guy. It's ingrained in our biology; we're geared at some basic level to reproduce even though we're moving ahead from that through the social structure we call society. That doesn't mean this impulse won't be there.
It seems that you want to date a girl too at some level and it's what you should be doing,if you decide you want to. Ask yourself why do you feel so bad about it? So guilty? Is it what your upbringing tells your or is it your peers?
By dating a girl you would be doing what humans have been doing for millenia. There are literally thousands and thousands of books and poems to attest to the fact that being with another person is one of the most profoundly beautiful things in life. It is one of the few really amazing things we're capable of feeling and doing as humans.
So listen we have to our heart and get out there, maybe ask out a girl whom we really like. Let yourself love and be loved.
How can we stop thinking about girls always?
It will be possible when we are thinking always about our family and our brother and sisters. When we see our sister in every girl in this world then It may be works that we can fully ignore the things working towards our life.
You know what when we are thinking about our sisters then we can't think anything wrong about her and don't want to listen anything about her.
So, When we are going to see our sisters in every girl of this world then it may be possible to stop thinking about girls always.
Start Loving Single girl with our pure heart then it can be possible.
As I love someone very badly but she can't with me. I am in relationship from 2013 with a single girl and my thinking about girls were always respectful.
Start respecting girls and women. Take this decision you can see the difference.
How to change our mindset about Girls?
Imagine yourself being treated like an object and derive the changes you want in others. Its not about being brother, friend or sister. Take a girl as a human like you are one. Hope this thinking may change you because we act like we think and we become like we act.