Is there a way to filter dog boarding sitters who are willing to watch unfixed dogs?

No, the filtering capabilities on the website are limited. Your best bet is to call Rover Customer Support. They have additional search filters and can find you a sitter. The number is [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team]. There is no additional charge for this service. Be sure to tell them they need to improve their search function. If you don't, they'll never know that customers are dissatisfied with the site.

August 2019 and experiencing the same problem, such a simple search option to add to the functionality, what a waste of time!!! Called Rover and guess what, the search doesn’t work for them either, they suggested I just keep trying different sitters, wow...

I cant believe its been over a year and rover still hasn't fixed this! Whats most frustrating is by clicking "Contact sitters who do" Users are fooled into thinking a filter is applied only to find out its not. That's on the site...the app is even worse! The app will actually let you message the sitter who doesn't accept unfixed dogs. It gives you the message but the Envelope icon remains active, fail to read the message and waste a whole lot of time both yours and all the sitters you contacted. I respect and support the sitters choice to accept or not accept the dogs but rover should have a filter!

This is terrible.

You should also make sure your dogs' profile is correct in regards to they are not neutered, https://www.rover.com/account/profile... the search results should eliminate those sitters who will not accept unneutered dogs.