First, there are things I talk about with my girlfriends that I just won't talk about with the boys. These issues range from, "Does this look OK?" to, "I'm on my period, I'm bloated and this SUCKS!!!" Fashion and biology aside, I find I want a female confidante, someone with whom I can just chill and talk about all sorts of things. While some of my male friends are sensitive and compassionate (and I do feel comfortable talking about things with some of them), they can't really play this role. You can't really have a slumber party with your straight male best friend, it looks and sounds completely inappropriate, even if all you were doing was chatting, watching a movie and eating popcorn. I also wouldn't find it appropriate to constantly confide in one guy when you're dating someone else. Sure, you can be close friends, go out for coffee, keep in touch, but there are just lines you can't cross. For me, I feel more comfortable having the "no-holds-barred" conversations with the girls.
Second, sometimes lines get blurry before you even realize it. It's great if it turns into a loving, romantic relationship that ends in marriage. It's not so great when you've been buddies forever and, all of a sudden, you have feelings and he doesn't. Or vice versa. That can make a friendship turn awkward. Of course, then it turns into, "I want to keep our friendship going," but this can prevent people from moving on because there's always that hope. Sometimes, everything turns out fine, especially if there are periods when you don't constantly have contact. It's harder when you see/talk to them every day. Also, sometimes it does turn into a romantic relationship only to end in a really bad breakup and consequent end to the friendship (though, given time, there is the possibility of patching things up).
I think it's good to have friends of the opposite sex. I think these friendships give us perceptions and insights that neither romantic nor same sex friendships can provide. Like I said, I love my boys! However, it is good to be prudent. You don't have to observe every gendered expectation out there (and I would tell you to challenge them). Hell, if none of this post works for you, don't follow it! Just always make sure you consider your own values, your own boundaries (of course, this should go for any friendship). For me, I've had some personal experience with these, which is why I'm writing this post. Good day, everyone!