My civil partner is British and we live in the UK. Even though we are not technically married, we always fill in one customs form as a family and go through the US citizen line when visiting the US, as I'm a US citizen. If you are married they definitely want you to go through together. Sometimes the people directing traffic don't really know--but consular officials have advised us that immigration officers want you to go through together, as then, they have the complete picture in front of them.
As at any border, just answer questions truthfully. You are American, visiting with your British spouse. My partner just gets the fingerprint treatment like anyone else with an ESTA, but at least we're at the same desk together and I'm not waiting for her for hours at the luggage carousel.
Have a great trip.