It's hard to not cross the line. Being friends with the opposite sex means spending tons of time together. However, you must know your boundaries of what you can and cannot do or talk about. It is quite common for the male or female in the friendship to have a crush on the other person. This leads to many difficulties. Although, if it's pointed out from the start that the relationship will not grow more than just a friendship, it may be a blessing.
A crush obviously means feelings for the other. So, when one is in trouble, they can go straight to their friend and talk them through it -- many times being more sincere than others.
Although this sounds great, it could lead to problems. If the friend keeps his or her feelings to himself/herself for a long period of time they could explode, leaving heartaches on both sides of the spectrum. Even if the feelings aren't kept bundled up, heartache may be in order once the other finds a partner. Which, may also end up hurting the partner, making them jealous or forbidding the friends to see each other.
Most friends of opposite sexes are great from the start, but fade in time. However, friendship may become more; many friends become spouses later on, be it ten or twenty-years down the road. Most just need to realize their true feelings for the other before becoming serious.