Our self-image has been hurt by homophobia and heterosexism. We have been told we just don't match up to cultural ideals. So, besides the usual unhealthy messages about the fulfillment of parenting that straight people use to parent, we might feel the additional need to live up to this straight standard. It will make us feel the fulfillment we don't have because we're not straight. It's not that same-sex couples cannot bring up healthy children.
Some studies suggest they do better than heterosexual couples.
It's not that children need both a father and a mother to be healthy. That's a popular cultural myth. If any human being is fully human, then she or he can model for a child what it is to be fully human. If a relationship is healthy, then it can model a healthy relationship. They are not here to fulfill us.