As thoughts go, that isn't a very smart one. Having a baby only requires sperm, not a man's direct involvement. Do you really think a heterosexual couple who can't conceive due to the husband's infertility would for a 3-way relationship with the man who donates sperm? I would sincerely hope your answer is no. A couple (lesbian or not) can have a baby through artificial insemination without ever meeting or knowing anything about the man the semen came from. Also, they do have the option of adoption of a baby rather than giving birth themselves. And as others have pointed out, a heterosexual couple choosing to not have children or not being capable of reproduction doesn't eliminate the "point" of heterosexuality, so the choice to have children does not eliminate the "point" of homosexuality and same-sex couples/marriage.
Children can easily adjust to the fact that one of the parents isn't blood-related to them when that's a fact of knowledge they were raised knowing. They may want to eventually find their biological father (or biological mother in the case of a surrogate mother giving birth to a baby for a gay male couple, or heterosexual couple whose female partner is incapable of having a baby). That has nothing to do with the matter of the parents being a same-sex couple.
You find it so strange because that is not the life and childhood you have, so it's not something you're personally used to. For children who are raised in such a household, it's every bit as natural and comfortable to them as a pair of heterosexual parents is for any other kid. And apparently your objection to it is driven by some sort of horseshit religious dogma, as evidenced by your reference to a god, which has fermented some level of homophobia and prejudice in you.