The difficulty in sanctioning one form of free treatment over another comes down to the correct distribution of funds, which dedpends entirely on from which aspect of the funding you are approaching the NHS.
If I was an infertile woman over forty, i would no doubt feel that my case is important, similarly if i was a lesbian looking to have a child, the same would apply.
The difficulty for those in charge is trying to stretch a shortfall budget in a way which allows moral and ethical choices to be accomodated.
I would not however sanction private treatment only for lesbians as this is a fundamental discrimination based on sexual orientation - if women over forty are given free treatment, then that's it - not only women who are heterosexual.
The argument that raises its head is that a large prortion of tax payers feel that supplying fertility treatment to older or lesbian women is wrong - which in my view is predjudice.
That said, I believe that conception is a gift rather than a right, and in my view, if nature has decreed that a woman is not going to conceive, then tragically, that is the situation they are in.
Easy for me to say - i am a\ man with three children - so i honestly have no simple answer to a difficult situation which is both financial and ethical.