"Why Should girls be alowed on boys sports teams???"
As long as they are not expecting or requesting special treatment then I see absolutely nothing wrong with girls integrated into boys teams. In fact it would just be better to state them as teams and not actually refer to them as "boys" teams.
There is always going to be a little bit of a backlash though because of ingrained social interactions. I remember seeing a documentary about two girls who joined a male high school football team. Some of the opposing teams stated they felt the need to hold back being as aggressive as they would normally be towards their male opponents because they didn't want to be accused of being too harsh. While other players suggested that if they wanted to play with boys they will get no special treatment and will have to deal with anything that would normally happen towards their male counterparts.
I remember when I was taking high school physics and we were dealing with the concept of center of mass of people and as it turns out women actually have a lower center of mass than males. Which in theory would make them better football players since they would be harder to tackle. Anyone who is familiar with football would understand this point. But I don't know if it really plays out that way due to other factors.