My answer, originally published on Quora.
They do it the way ANY adoptive parent -- male or female, gay or straight - does it: usually -bottle-feeding with formula.
Entire generations of kids grew up healthy on just formula, and today formula more closely resembles breast milk than at any time before. My kids - both adopted - were exclusively formula-fed until 6 months of age and they are healthy, developmentally on-target (or ahead of target), properly bonded, etc.
Some adoptive parents also supplement formula with breast milk obtained from a regulated donor milk bank (http://www.hmbana.org/), from private sellers of breast milk (example: http://www.onlythebreast.com/ or craigslist), or by seeking donations from friends and family.