Sorry not period or sex related but I really need advice!

So my fianceé and I are getting married next June. We are being VERY selective with the guest list because we are paying for it ourselves and are just two broke girls. Since we are being selective we've made a guest wide decision that there will be no plus one's unless someone's significant other also happens to be friends with us or close to us. We only want people that truly matter to us at our wedding. Only people we truly love and love having in our lives. Unfortunately, my mother's boyfriend is not one of those people. I know it may sound a little selfish, but to be frank part of me doesn't care, I don't want him there. The first few months I knew him any time he spoke to me he was incredibly rude. He was the same way with my fianceé. He makes me, my fianceé, and my little sister uncomfortable. And then I have a bunch of petty reasons for disliking him that all build up into me loathing this dude. My budget is slim for what I want to be a very elegant wedding and reception. So the idea of spending the money on a plated dinner for someone, regardless of who they may be, that I nor my fianceé like....that really doesn't sound appealing. I've seen on wedding blogs and stuff say that the "I'm the bride this day about me" thinking is illogical and the moment you invite people in the celebration is for them....but yea no this wedding is about me and my beautiful bride.

So I guess what I need advice on is how do I delicately handle this situation with my mother. Although I adore my dad's girlfriend, we won't be allowing her to come either to make it fair. And like I said, no one gets a plus one. I'm afraid she's going to throw a fit and say it's unfair or even threaten not to come....which to be honest if she did I think I would just say 'okay'. My mother and I don't have the best relationship.

*TLDR: I need some guidance. I strongly dislike my moms boyfriend. How do i tell her i dont want him at my wedding. And please don't tell me it's selfish because I know..*

I'm sorry if this was the wrong forum to post this