The MIDUS Study is a recent sample (2004-2006) of middle-aged people which makes it useful to measure how many kids gay men and lesbians are having in an era in which homosexuality is widely tolerated. The limitation of the study (in addition to small samples of homosexuals) is that participants were asked to count stepchildren, adopted children, and foster kids in addition to biological children. Anyway, here are the averages for people 40 and over.
Mean number of children (N = 3,521)
Straight men have 6 times as many children as gay men, but heterosexual women have only 1.25 times more kids than lesbians. Bisexual men are in between straight and gay men, but bisexual females are the least fertile women. I'll warn once again that non-biological children are throwing off the numbers.