So it is up to you to decide what is best for someone else's child?
The psychological research that claims that a single motherhood prevents unfit offspring is the same group that believes that as long as there are two viewpoints of the world (A stern figure, and a tender figure) attribute to a healthy growth environment. They have actually stated that it does not have to be two people of opposite sex. It's the emotive value that is in concern.
So, effectively, if you have two stern parents, and noone to offer soft emotions, you will potentially have a child who is "unfit" for the "normal" society... But they'd make a great soldier!
The best things in life go on sale sooner or later.