Thoroughly agree with Cecil that honesty is the best policy. However, in the gist of science itself not being a bad idea, just what you potentially do with it:
How about Chlamydia? (Chlamydophila now, before some smarty-pants corrects me). The OP presumably has no issues with deception so sleeping around may not be a concern. The benefit over the mumps is that Chlamydia is symptomless (other than the infertility) most of the time, whereas mumps orchitis apparently hurts like hell. It’s also easily transmissible, so if Mrs OP starts playing away in an effort to get pregnant there’s still some protection (quite how often and how effectively chlamydia decreases fertility I don’t know). Downside is that you don’t know what else you’ll pick up while fishing for Chlamydia.
Alternatively anabolic steroids are fairly readily available on the black market and cause your testicles to think “well, I’m obviously working too hard” and shrivel. Not sure what it does to sperm count but I suspect it’s not good - it can also screw with libido which could be an effective way of achieving the same end result. Add in some gym time and it’s not like Mrs OP gets no benefit from the deal (but make sure she does actually like a man with breasts if you ever decide to stop). Insert list of long side effects here.
Lastly there are a number of injectable GnRH analogues (google suprelorin), developed recently to medically sterilise dogs. It’s since been licensed for ferrets and I believe it’s been shown to work in cats. There might be a very good reason why it isn’t so far available in humans though - I imagine there would be a certain market for it if it was safe and effective.
The implant itself is put in with a big needle and is about the size of a grain of rice; it lasts somewhere between 6-18 months dependent on the species. One downside is that it is often palpable under the skin, and if you put it in the arm it feels sufficiently like the progestogen implants that some women use that that would probably be the first conclusion Mrs OP would jump to. If you inject it into fatty tissue it might not work as well.
