Does sex ruin a relationship?  If you value your friends, is it not risky to engage in intimate relations where the consequence could be altering or losing the relationship totally?  Is this not a risky proposition where someone will get hurt?  http://blogs.psychcentral.com/healing-together/2012/06/friends-with-benefits-or-friends-with-complications/

It seems that many people believe that it IS possible to have “friends with benefits”, but this article is correct in many ways. Expectations are never clear; nor are they the same for each person. And, some people think they can handle the ramifications of engaging in  it, but then find out otherwise. They become emotionally attached and start wanting more from the other person.

Definitely, there is a need for consideration on all counts. You really need to think about the consequences and the complications of changing the nature of your relationship, especially if you want to remain friends for life!