needing a mother and a father

While it feels like gay marriage is well accepted by most American society, official LDS doctrine does not tolerate it.  A common argument against gay marriage is that having both a mother and a father is best for children; they "need" one parent of each gender.  If that's true, then mothers and fathers play truly different parts, and that gender roles are a real and important aspect to the parenting triangle.

Obviously, the LDS church is pro-gender roles, advocating them in teaching children and in the general structure of the family.  But the roles are not well defined: in the eight-thousand-plus word chapter on teaching gender roles, roles are defined as mother/wife and father/husband, and then implied elsewhere along the lines of "each child is learning how to be male or female and about what being male or female means about their relationships with others."  Nur?

In the family proclamation, we're given a sliver of insight as to what they actually mean: men are responsible for providing and presiding and women are primarily to nurture, but that men and women should be equal partners as parents.  This has been picked apart from every perspective, but it's still pretty opaque.

The message I'm getting is that gender identity is important, even if we really have no idea what accompanies gender.  And this brings me back to my original point: if gender defines very little about a person's personality and parenting style, then why is having both genders represented in parents important?

Admittedly, it would be harder for a girl to have two male parents, or vice versa, because there would be social and biological questions that may be more difficult for an opposite-gender parent to answer. Harder, but certainly not impossible.

The irony here is that in the LDS church, we only really have father figures, or male spiritual role models: Heavenly Father and Jesus Christ.  There are stories about more men and women, but we're supposed to become like these two figures.  We're not supposed to be like Mary the mother of Christ.  I mean, we are, but only in as much as she is like Christ.  And we're not supposed to try to be like the only-whispered-about Heavenly Mother, since we have no idea what to emulate.

You can't have it both ways.  Either you need both parents, especially for spiritual guidance, or else gender in parents doesn't matter.  Either we have a Heavenly Mother and Her role is clearly defined (preferably with an accompanying Matriarchal Priesthood), or gay marriage is okay.  Which one is it?

An addendum: I didn't talk about single parenting, which is another situation that the LDS church doesn't handle as well as it could.  No matter the doctrine of the church, alternative family structures need to be more welcome, which is a job for the members and not the hierarchy.