From the article:
While gay and lesbian Americans typically become parents today in four ways—through one partner’s previous participation in a heterosexual union, through adoption, in-vitro fertilization, or by a surrogate—the NFSS is more likely to be comprised of respondents from the first two of these arrangements than from the last two.[...]
Although the NFSS did not directly ask those respondents whose parent has had a same-sex romantic relationship about the manner of their own birth, a failed heterosexual union is clearly the modal method: just under half of such respondents reported that their biological parents were once married.
So, they are not necessarily biological parents who had homosexual affairs but predominantly that is what they are. They may or may not be gay couples raising a biological child of one of them; just as likely, they are married parents with an homosexual affair on the side and the child being with the straight parent:
Among those who said their mother had a same-sex relationship, 91% reported living with their mother while she was in the romantic relationship, and 57% said they had lived with their mother and her partner for at least 4 months at some point prior to age 18. A smaller share (23%) said they had spent at least 3 years living in the same household with a romantic partner of their mother’s.Among those who said their father had a same-sex relationship, however, 42% reported living with him while he was in a same-sex romantic relationship, and 23% reported living with him and his partner for at least 4 months (but less than 2% said they had spent at least 3 years together in the same household), a trend similarly noted in Tasker’s (2005) review article on gay and lesbian parenting.
(all quotes from pages 756-757).