I never thought I'd wade in to such a discussion but here's my two penn'orth.
A lesbian (or gay) couple, fundamentally, cannot biologically conceive a child without help from a donation from the opposite sex. And said child would only be genetically related to one of the couple. There's nothing wrong with the biology, there's no medical intervention available that can change this, it is just a fact that a lesbian couple cannot conceive their own child. Should they seek a donor, and the child enquire about their father in the future, they will have no answers other than 'we did this (ie deprive you of a father) in order to satisfy our own needs'.
A heterosexual couple fundamentally CAN conceive, and if there are faults with the biology, there can be medical interventions which will make conception possible. The grey area remains as to the nature of the donor/donation as well, I suppose, as the 'we did this to have a baby', but the fact remains that there is usually something wrong in the biology that is preventing conception, not that it is impossible (as in the case of lesbian parents).
I say that coming from a position of my own dad not being around much, and likewise my kids dad has reduced contact to Xmas and birthdays, so I see the impact that a missing dad has on people.
In OP's case, she only has sole care of her dc 4 nights a week. She can support her current dc. Adding another without another parent around is going to put pressure on her, both her parents, and inevitably reduce the time, emotion and cold hard cash available to the existing dc. Before you ask the inevitable question of 'why don't I have a daddy like everyone else?'