Can girls and guys be best friends?

Now let me start with I have many female friends of my own, but my gf has a guy bf who she hangs with at least once a week. Now I'm genuinely not jealous and didn't care at first because I trust her 100%, but iv seen soooooo many of my own friends and herd so many stories aswell of people who have had their partner cheat on them with their so called "childhood friend". In saying that she's the most genuine and honest person I know but I have not seen any evidence of opposite sex friends that have not slept together, especially since they are extremely close. Am I over thinking it?