Yes it is fair as long as everyone involved knows everything....no secrets.....and who knows - maybe the 2 will want to share you !! :-)
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.