How can I, a 28-year-old female, communicate to my 26-year-old male partner that I'm feeling uneasy about his mother-in-law, who is 47?

**Trigger Warning: Discussion of abuse** Hello everyone, I find myself in a challenging situation and could use some guidance. I’m using a throwaway account to keep this private from my partner. To provide some background: I (currently 28F) experienced abuse between the ages of 17 and 20 by an older man, which has left me feeling very uncomfortable in relationships with significant age differences. Currently, my partner (26M), whom I'll refer to as "R," and I are in a polyamorous relationship and practice Ethical Non-Monogamy (ENM). We generally have an understanding that we can date and sleep with others separately. However, the other day, while we were driving, he asked me to check a message thread for him. When I opened it, I noticed a message where he referred to someone as "mommy." R explained that this was a woman he was planning to meet and potentially sleep with, whom he described as a "MILF." We typically don't talk much about our other partners, so I immediately felt uneasy and asked a few questions. He mentioned that she is in her late forties. I requested to see a picture of her, and I commented that she didn't seem to be my type, probably because I'm not attracted to older women. I don't want to come off as hypocritical since I also have a casual relationship with a 35-year-old man. Later, as we were lying in bed, R asked if the age gap between him and "his MILF" bothered me. In the moment, I wasn't sure how to respond and simply acknowledged that he is an adult capable of making his own choices. He knows about my past relationship and the trauma I carry regarding age gaps. He assured me that he only plans to see her casually, but I still feel highly unsettled about the whole situation. I welcome any and all advice. Thank you.