"Despite the arguments
that same sex marriages is
stating it is
impossible by definition, a personal lifestyle choice, and encourages a bad environment to raise children, same sex marriages should be legalized because people are being denied both equal rights and the separation of the church and state; additionally, same sex parenting has not shown significant negative impacts on children."
I fixed it up, above, but I think the teacher is right about it being too long. You can explain the reasons people argue against it later. For the thesis sentence, just write your 2 reasons for arguing in defense of same sex marriage.
This statement is already good: "...both equal rights and the separation of the church and state." People are being denied both of those ideals, so I think you wrote it will. It is not a place for parallelism.
I always think you should take the teacher's advice, because taking advice is a way to show respect and appreciation: "Same sex marriages should be legalized because people failure to legalize it denies people equal rights and interferes with the separation of the church and state."
You have a 3rd reason, too -- that it does not negatively impact parenting -- but you can mention that in the paragraph where you refute the counterargument. Do you know what that means? In the second-to-last paragraph, essay writers sometimes explain the opposite argument and why it is wrong... so that is a good place for your observation that same sex marriage does not negatively affect parenting.