if s/he's a friend you don't add sex.
A friendship has love, but sex would be taking a risk and crossing the line and can destroy the relationship. If you are in love with a friend, proceed with caution, because if he is not feeling the same way can result in ruining the friendship, awkwardness and discomfort around each other and can break up the friendship, so proceed with caution, because if he is not responding or feeling the same way, you can lose a friend.
If you are having intimacy with a "friend", the line has been crossed and only time will tell if it will lead to a high level in the friendship to exclusivity, but be very careful with a friend and crossing the line to intimate friend, because you just might end up hurt and alone or a possible relationship.
Sometimes friendship is all that it is for the other person and while you (apparently) might want love and sex, the other person is not interested in anything but friendship...you cannot force it to change. Making that attempt could ruin the friendship. you haven't given us any info on how the other person might feel, so I'll guess that is all he/she is interested in..Do you want to maintain the friendship? Don't try to push the romance.
Jim