I'd take Amethyst's advice a little farther. Warm up with 1 finger, then 2 - then try a tapered toy like a butt plug that will max-out at 1.25" to 1.5". Once you can handle this without a lot of discomfort - then lube up you and your partner and give it a try.
When we first started I would penetrate my wife - and than have to pause for her to adjust. Then it was slow short strokes - the more she adapted the more we progressed.
One good way to do this is either for you to be on top to control the speed and depth of penetration - or in doggie style - again with you controlling the speed and depth of penetration.
As a couple once you master this - you will find that eventually you will be able to enjoy your partner thrusting as he normally would during PIV.
It took about 2 months of fairly regular anal sex for my wife to enjoy my thrusting - I can tell you the patience was well worth the reward for both of us.