Do I need to show a plane ticket when applying for a re-entry permit on a marriage visa in Thailand?

No

For the amount they cost, I would say it’s more convenient to get a multi-entry permit rather than a single entry. It would give you peace of mind. If something occurred elsewhere, while you are in Thailand, you just need to buy a ticket out, go to the airport and leave. You don’t have the added distraction of getting a re-entry permit. Anyway, the decision is yours and the costs are:

single re-entry = 1000 baht

Multiple re-entry = 3800 baht

Simply put: your re-entry permit is valid for the period left in your extension. If you buy a multi re-entry you can leave and re-enter multiple time during the life of the extension. If you buy a single re-entry permit you can depart and arrive once during that period as the single re-entry expires when you re-enter Thailand.

I got my multiple re-entry at my local Immigration office at the same time I got my Non-O extension based on retirement so I can’t comment of getting it at the airport. However I am aware that the marriage extension has a lot more hoops to jump through before it is granted than a retirement extension so I would bear this in mind with regard to your return date lest you not have enough time to renew the extension.

I leave for 6-7 months . Only need to get a reentry permit at the airport. I get my extension right before I leave

You don’t need to show anything. Just get reentry permit. Otherwise, your O visa expires when you exit Thailand. You can do it in the airport before leaving.

The most important thing is you must return before your extension of stay expires

All that's required

Just put some dates that you think you may travel on. No ticket is needed.