That would be a decision between the bio-parents (or guardian) and the couple. In general terms I believe that both genders are needed and one can never substitute for another as they contribute different attributes to a child so a m-f couple would be in general preferable.
Just going on a line of thought, a f-f couple might be the next in order of preference as one female may take on the full maternal role, which IMHO is critically important in early life. A single f may not have the time required to nurture a baby, though there is a greater possibility of the addition of a male in marriage for the father role later in life. A m-m couple would be the least preferable IMHO (of the 4 possibilities mentioned), as the father role, though very important for normal development, is not as critical as the maternal one at the early stages IMHO.