Depends on how you define a friend but overall or at least by my definiton of a friend they can't be. There is always sexual tension going on if men and women are together, it's biological. You might have noticed that if your guy friends act different when you or another girl is around, it's normal. So going further with that framework in mind they maybe can be "friends" but it'll always be a bit different with their male friends because of the point above. But of course there are always anomalies so if it worked for you and a couple of others, great!