Yes it is fair as long as everyone involved knows everything....no secrets.....and who knows - maybe the 2 will want to share you !! :-)
Since you didn't use the word "friend", something is going on and it's not fair to any of you.<br />
*very unfair to your "love" - if you do love them, not sure you do if you can do this<br />
*unfair to the one being used, even if they know<br />
*not fair to you if you admit you have a problem - you cannot be alone - and don't try to improve your strength and character and patience. Especially if you do love someone else - because you are setting a precedent, setting yourself up for trouble down the road and more hurt for your "love" and for yourself.
If both the parties know the truth then its fair.
that depends upon your conscience. Turquoisebluelight is correct that honesty absolves the person from liability if any harm results, but fairness is a subjective matter. that sort of thing would not be easy for a person such as myself, who feels that ultimately risking harm to another in preference of satisfying my own selfish (albeit temporary) needs, is neither right nor fair to the person who is being used.
ALL is fair in love and war, remember?