If this hypothetical lesbian doctor truly believed that heterosexual men and women should not produce children with one another, it would be immoral to force her to help a straight couple to conceive, or to punish her for refusing.
As somebody said earlier, a doctor is not a slave.
The problem I have with this whole thing is if she had undergone procedures in the office to help her become fertile, the doctor knew her intent. He should have denied her up front. There are clinics that state up front they do not cater to homosexual couples or single women. But to introduce a 'policy' or belief after someone has already received infertility treatment seems highly unethical, not morally upright.