I understand the reasoning behind the whole "Child that is artifically inseminated / Adopted by a gay couple argument" would potentially hinder a child's maturity / growth, especially if said child is the opposite sex of his / her gay parents, and then issues for the child may come up (Can two dads help a girl prepare for her first period? The child may wonder "If both my 'adopted' parents are gay, maybe am I gay too?")
But wouldn't these issues also come up if 2 heterosexual parents adopted a child / did artificial insemination (say the husband is unable to reproduce because of a groan injury, and the wife must find sperm elsewhere), or one of the parents dies and it happens to be the gender of the child so the child is left with the parent of the opposite gender (the scenario about the girl worrying about her first period would again be relevant), but this time in a heterosexual coupling.
You pointed out the RI Lesbian scenario, well what about hetero couples who want a divorce after 3 years but for some reason feel morally/religiously obligated to feel divorce is NOT an option, so although they can get divorced they do not. Is that any better? They both want to feel their "divorce would be honored" but they themselves feel that a divorce is not honorable, or perhaps even if they don't mind they know their families would mind for the same reasons. In other words, saying a marriage only happily lasted 3 years does not justify preventing an entire community of Millions of gays and bisexuals around america from getting married. Gays are about 3% of America's Population, so of the 320 Million Americans that means 9.6 million gays/ bisexuals cannot get married.