This assumes the bread is consecrated, as in the eucharist. As I see it, if Catholics believe that:

Sex within marriage is allowed, but outside of marriage is sinful; Nuns are married to Christ; The eucharist transubstantiates into the literal body of Christ;

then it follows that they should be able to have sex with said bread.

The only possible counterexample I can think of is that procreation is impossible via sex with bread, but, from some Googling, it appears that Catholics are still able to have sex within marriage after conception is no longer possible (i.e., post-menopause) as long as they do not actively try and prevent conception (source [here](https://www.ncregister.com/blog/sex-and-catholics-our-views-briefly-explained)). I can’t imagine an objection based on non-monogamy given the inherent non-monogamy of all nuns being married to Christ.

Please change my view, this thought is haunting me.