Poly isn't something you should even think of attempting to please someone else. Chances are it would just end in resentment. If your heart isn't in it just don't go there. If it means the end to the relationship you're in that just means you life has opened up to someone who fits your needs better.
And now I'm going to totally go off on a tangent: Why is it that in relationships where both partners are straight and they want to add a third is it always the woman who is expected to go against her natural sexuality? How about asking your boyfriend how he would feel about adding another boyfriend for you both to share? If he wouldn't do it why should you? If you're not into someone you're not into them and pretending to be will just be hurtful in the longrun to that person!
Everything will be ok in the end. If it's not ok it's not the end.