Monogamous ladies, how do you deal with knowing that your SO fantasizes about having sex with other women (that he knows)?

First off, I'd like to apologize if this is a question asked all too frequently. I've searched Reddit for similar threads and found some, but not many of the comments were particularly helpful for me. I'm hoping to get some fresh answers here.

A little background about me (F/21) and my SO (m/23): this is my second relationship and my first serious one. We've been dating for about 8 months, so the relationship is still pretty young. We've had our ups and downs, and have recently been getting closer and building our relationship. I have trust/insecurity issues because I watched my mom suffer a very brutal divorce (he was cheating on her), and these trust issues were exacerbated last December when I caught my SO flirting with a former crush of his. It was devastating to me, but we decided to work it out and after a couple of rocky months, things have begun to go well and I am learning to trust again.

However, we were laying in bed today after sex and somehow the topic of fantasizing about other people came up. (To make things clear, when I say "fantasize," I mean actually conjuring images up in your mind while masturbating to them. Not just thinking, "I'd do her.") Now, I know that many people fantasize about other people while in a committed relationship and that it doesn't necessarily mean that they're cheating, but I still have problems coping with this. I'm aware that my trust issues and insecurity contributes to this. I'm pretty sex-positive and know that humans are sexual creatures and may need variety. However, to me, I think that to fantasize about people that you know in real life kind of oversteps the boundary...kind of like emotionally cheating? If he just wants variety, why not look at porn and pictures of naked strangers? Myself, when I do fantasize, I fantasize about 'faceless' or 'unknown' entities. That is, I don't use anyone that I know; I just make someone up, because my fantasies don't actually focus on who that other person is, but rather what they are doing to me and how that turns me on.

For those who believe that fantasizing about people you know is okay, is there a line? For example, is it still okay if he masturbates while fantasizing about other people 90% of the time as opposed to 10%? Is it still okay if he fantasizes about one or two people in particular as opposed to a bunch of different people? What if he fantasizes about other women while having sex with you?

I think in the end, it all comes down to different people having different expectations. My question is, how do you deal if your expectations differ from your SO's expectations?