Noun, Combined term fraternal (brotherly) and romance
A Framance is a strong relationship between two males where one or both of the males are straight and they are having sex for mutual convenience and pleasure. Any outside observer would consider their actions to be more like a gay romance and gay sex, however, long term the intimacy of the relationship will most likely end when one of them begins dating or marries a woman.
A similar word Bromance is a portmanteau of the words bro or brother and romance. Editor Dave Carnie coined the term in the skateboard magazine Big Brother in the 1990s to refer specifically to the sort of relationships that develop between skaters who spent a great deal of time together. However, this word was intended to refer only to a platonic relationship with no sexual element.
When a straight guy agrees to have a sexual relationship with his gay friend, it is important for the gay man to keep in mind framances such almost always end when the straight guy finds a girl friend.
The special relationship between a pair of twins that has a sexual element of intimacy is best described as a framance.