So you want to fuck your wife in the ass, but her asshole is too tight for your dick. The **** is practice, practice, practice. With proper practice just about any asshole can open up to take just about any dick, and even things a LOT larger than a dick. Look up fisting on the internet and it's not difficult to find photos of guys (and sometimes ladies) taking a hand and arm elbow deep in their asses. If these guys can stretch out their assholes this much, your wife can stretch out hers to take your dick.

So how do you motivate her to take your dick in her ass? Let her fuck you in the ass with a dildo. Get a dildo that is at least as big as your dick (and preferably bigger) and let her see that you can take it.