Look out, she might be trying to cover a pregnancy she already has from someone else

Get a vasectomy.

So get the snip already, chief.

I got a vasectomy when I got engaged, because we agreed not to have children, and we didn't want to worry about birth control failing. If you're sure you never want kids, it's quick and painless; and if you want, you can specify that you would like it to be reversible, in which case they can do a slightly different procedure and you'll wind up with a pretty high (but not guaranteed) chance of being able to change your mind later. (Note: that was 24 years ago for me, so it's quite possible the technology has changed.) However, I don't think this is something you do on your own if you're married. You and your wife need to agree on whether children are in your future or not, and if you can't agree, then you should end the marriage now, so that both of you have the most amount of time to find new partners. Also, I'll echo what other people have said: if you frequent forums on infidelity, sometime women really do get pregnant through an affair and then try to have sex with their husband as soon as possible, even if they've been through a long (multi-month or even multi-year) dry spell. I don't think it happens very often, but it happens often enough that I've seen it reported at least a handful of times.

Just go get fkn sterilized yourself then.