Actually...you can follow up and make sure the procedure is done. Yes, it is better for them to be in a home then on the streets..but an intact cat has a greater chance of being returned to the street because: spraying, howling, pregnancy, fighting etc. etc. etc. So the best thing to do is to give all your adoptors the following links:All I can do is hope that the new owners get the cat fixed which I do express this to them.
www.friendsofanimals.org
http://www.lovethatcat.com/spayneuter.html#mi
I would also have them all sign a contract that they WILL spay/neuter and I would give them a timeframe and follow up. If you need a contract, you can go here:
http://www.natewood.net/Rehome.html
and download the PDF called "Finding homes for homeless pets"
Katie
P.S. I would at least charge a small fee....perhaps even $20 that can be returned to them once the spay/neuter has been performed.