The dynamics of a boy/girl friendship can be complicated already. Friendship, by definition, is a relationship between two people that share a mutual respect and concern for each other. This alone involves some degree of intimacy. If you develop feelings of a sexual nature toward your friend and you act on those feelings, that will change your entire relationship.
A true friendship is a very open relationship. You share a closeness and understanding that already resembles some form of love. Friendship involves several emotions and a very sure sense of trust. You have to be able to talk openly about a variety of things and feel you will not be judged. And, in turn, you listen to your friends experiences and be objective on any subject.
Sex, for most people, is a very emotional experience. If you add that much emotion into a friendship, you are asking for trouble. You will loose some of the openness you used to have. You won’t feel as free to talk about certain things, and you will start to feel self conscious around the person you used to think accepted you no matter what. Then, you have the chance, and a very big chance at that, of one of you falling in love with the other. If the romantic love is only one sided, then jealousy will follow. As we all know, jealousy will ruin any relationship, even a strong friendship. Just think about how you would feel if you fell in love with your friend, and a few months down the road they fall in love with someone else.
Love born from a friendship can be the most fulfilling and lasting romantic relationship you will ever have. This is not true of a “friendship with benefits”. I believe if you are developing feelings of this nature for a friend, you should explore them within yourself before you act on them. If you and your friend have already decided there is a mutual attraction, set some boundaries, and address the most pressing issue up front. What do you expect and what is expected of you in this new sexual relationship? If you or your friend have no intention of being monogamous, then be honest about that from the beginning. Taking the time to discuss these things may put a stop to a potentially damaging decision.