Your wife should in no way feel obligated to try anal sex.
Some couples find it to be an enjoyable part of their repertoire – but by no means does having or not having anal sex have anything to do with an overall pleasurable and satisfying sex life. It is not a common form of sex within most marriages (statistics range somewhere between 9-30% as to those who have tried it at least once). I would caution against getting caught up on what a spouse is not willing to do – but rather enjoy the styles that your spouse is willing to engage in. As part of being open and having healthy sexual communication, it can be beneficial to let our spouses know our sexual interests and fantasies – but by no means does being open about something mean that now the spouse should feel pressured or responsible for making those interests and/or fantasies realities.
I will say that for those who do want to incorporate anal sex into their lifestyle – precautions should be taken (i.e. use of lubrication, in-tune communication as to stopping as soon as anything feels painful or uncomfortable, etc.).
I want to be clear that as a sex therapist, anal sex is never something I prescribe or recommend. If a couple wants to explore this sexual style and both feel comfortable doing so, then I help educate about how to do so safely. But it is not something I feel necessary to try in one’s lifetime nor would I want anyone feeling regret that this was something they never tried.