In one of the better studies of the effects of lesbians as parents of sons and daughters, researchers reported that 17-year old boys raised by lesbians were no more likely to be gay than those raised in straight homes. Gartrell, Bos and Goldberg found that 5.6% of boys raised in lesbian households reported sex with other boys whereas 6.6% of boys from a representative national survey reported ever engaging in sex with other boys. The difference was not large enough to be considered a statistically significant finding.
Reparative theorists claim that boys who are raised without a strong, salient father often become homosexual. In this study, the boys of lesbian parents had not been raised with any father figure and yet they were no more likely to report a gay identification than boys surveyed in a national sample with predominantly straight parents. If the absence of strong male role model generates same-sex attraction, the effect should show up in this sample.
I need to add that the group of lesbian parents represent a convenience sample and may not be representative of all lesbian parenting. Even so, the fact that boys raised in these homes displayed no behavioral indication of the effect predicted by reparative therapists is worth noting.
I assume these researchers will continue to follow these families and the results may shift more in line with reparative expectations. However, at present, this study is a challenge to the classic reparative theory.
Gartell, N. K., Bos, H. M. W., & Goldberg, N. G. (2011). Adolescents of the U.S. national longitudinal lesbian family study: Sexual orientation, sexual behavior, and sexual risk exposure. Archives of Sexual Behavior, 40, 1199-1209.
For more on this study, see this post. I should also make clear that this post is not intended to be a comprehensive review of this study. I am here highlighting one aspect of it. There are many findings of interest, including the results with girls which indicates that girls are more likely to engage in same-sex sexual behavior.