I am writing this after reading this CBC.ca article:
There's no single right or wrong answer that can be applied to every possible case of a female athlete wanting to participate in a team sport where the rest of the team is male.
The obvious argument against having girls on a boys team is the physicality aspect. So in some cases, it depends on the sheer brutality of the sport.
As a counter to this, it also depends on how physical the female athlete's play is. Hayley Wickenheiser played hockey in the second-tier Finnish Elite League, and got along just fine.
As a general guideline, I would say this:
Allow the female athlete who wishes to play on the boys' team to participate in some full-contact practices with the team. If she appears comfortable with the level of physical contact involved, she should be allowed on the team.
This is perhaps a difficult evaluation process, but I think that as a general guideline, it would be a good baseline to start from.