“I’m tired, I might have mono.”
Once you get mono, you can’t get it again, so it may be worth it to you to buy yourself some time by contracting mono. Sharing water bottles with prostitutes or your more adventurous friends should do the trick. You could go with a serious ailment such as tuberculosis, however, this could quickly lead to pity sex, which you'll want to avoid.
“I’m on your period.”
Start laying the seeds for this now, make little jokes about “cycling together.” It wouldn't hurt to splash some fake blood onto your underwear, though definitely expect questions from the Mrs. if you go this route.
“I’m getting a call.”
Fake a phone call. Phones can be easily concealed in the folds of your pajamas or in the confines of your small intestine. When she asks who it is at this hour, just tell her it's work. When she points out that you work on the sales floor at Office Max, respond with something about how you hate when she says “Office Max” like that. For best results, make it into a big thing about how she's never supported you in your career.
“I have to use the bathroom.”
Go to the bathroom and return to bed. Continue until she falls asleep or dies of old age. This is just like your classic “Tortoise and Hare” situation. It'll take a little longer, but in the end, she will either be asleep or dead. Either way, no sex.
“I have a family emergency but it's not anyone you know.”
Consider starting a secret second family for instances such as this. Of course, if you need to get out of having sex with your second wife you’ll need to either use your first family for excuses or start a secret third family, which would be a bit inconsiderate to your first two families.
“I’m not in love with you anymore.”
This is more of a first-parachute-didn’t-open move, though effective if deployed right away as well. Again, you'll want to craft a few solid one-liners to back yourself up if you choose to field questions. And don't say it in a sultry, roguish, Bradley-Cooper-y type of way either, because this too could lead to intercourse. Think more along the lines of Steve Buschemi.
“I think I might be asexual for you.”
Sexuality has never been more meaningless than today, there are no rules and if she denies this than she is breaking the law. Remind her of this fact and quote think-pieces from VICE and NPR where necessary.
“It’s against my religion.”
Get serious about a strict religion now to make this more believable in the moment. As far as picking a religion to become fervently adherent to, most types Christianity are good, though Mormonism is probably best in this instance. Or go another route and join Scientology, in which case her sex drive should dry up pretty quickly on her own.
“I’m really busy with my passion project.”
Find a project, be passionate about it, let it consume you until you have no time or energy for sex with your wife. Just don't pick something creepy like model trains or ventriloquism.
“The dog ate my penis.”
“The dog ate my homework” is a classic for a reason. At the very least it's worth a shot. Once again, expect some fact checking on her end and prepare accordingly.