Our surrogate is threatening our marriage

We have spent 10 years in marriage without a child. My wife and I tried all possible ways to get a one but in vain.

We therefore agreed to get someone to carry the pregnancy on our behalf.

My wife introduced me to a lady who agreed to conceive and carry the pregnancy at a fee. I had sex with her and after a number of times, she conceived. She then requested to stay with us as we take care of her during pregnancy which we agreed.

Everything seemed okay at first but of late she started pestering me for sex claiming to have developed feeling for me. She is so disrespectful to my wife but we can't confront her because we are afraid she might refuse to hand over the baby at birth.

What should we do?

Robert