Not sure if it's a trend or just an interesting reconfiguration of the family, combined with a largely undefined notion of gay fatherhood, but The New York Times Magazine has a looong article about how some lesbian couples are kids using sperm from gay male friends.
Those biological dads then take on varying degrees of parent-style involvement that don't necessarily match with the donor agreements/contracts that often accompany such babymaking.
One constant condition, though, of the NYTM families seems to be that the men relinguish their legal biological parental claims. Whatever the man's involvement, in the eyes of the moms, the kid is theirs.
Chalk it up to the conservative family environment in Minneapolis and New York where the Times reports come from. Meanwhile, in San Francisco, the traditional lesbian family is under siege by a gay dad who moved in with his lesbian friend to raise their kid. Liberal hippies.