Then you are arguing that same sex couples should never be allowed to have children,
No. But as with opposite sex couples requiring a sperm donation, there should be process for proper transfer of the responsibilty for the child. It is fine to transfer the responsibility from the biological father to a social mother. But it should not be possible to just remove the responsibility of the father without providing a second person that steps in.
because it will always require someone of the opposite sex to assist with creating the child
I would not be completely sure about this. It might be possible at some point two eggs can be fused together giving birth to a girl with two biological mothers and no biological father.
and if that other party will always be a legal parent of that child then same sex couples can never really have children legally. That is wrong.
This does not really follow. If you do not allow to transfer the legal parenthood from its biological parents to social parents then same sex couples can not be legal parents of their social childern. But it would just the same for any opposite sex couple that requires a sperm donation, they could also not have kids. I do not see any reason why states must allow transfer of biological parenthood to social parents. It more seems to be something that is nice to have but I do not know of any fundamental human right that would be violated if you do not allow this.