To me, it sounds like there is just not much you can do, at this point. You are already broken up and there is no telling what the future will bring. Perhaps he will come back and you’ll want to take him back and that will be what happens. If he did sleep with others you’ll have to deal with it.
When I was in middle school, I met my first boyfriend. We had a very dramatic relationship, especially for our ages. He had dated another girl the year before me, and they broke up the summer before we got together. We started dating that fall, and two months in, he broke up with me and went back to her. It sucked and I was blindsided. Two days after we broke up, he broke up with her and asked me if I wanted to get back together. I was young and in love and just wanted to move past it, so we began dating again. Five months after that, he told me we should break up as we’d be moving on to different schools and lived far apart, couldn’t drive, etc. He didn’t feel he could do long distance. I was really sad, but didn’t have a choice. I suggested we break up on the last day of school. He fed me a line about how that would be so sudden and he wasn’t sure he could handle just being together one day and then not having me in his life at all the next day. So we broke up. We had never really been friends before dating so I stopped talking to him and just mourned privately… he bugged me about it one day after school until I told him I was just sad and missed him, etc etc. He told me he loved me so much and would always love me and probably not get over me for a long, long time. And so on and so forth. I was really touched. The next Monday, I found out he was already back together with the other girl. Lol. So much for not getting over me for a long time!! They dated all summer until she moved away and he had the audacity to call me and tell me he made a mistake and wanted me back. I told him no thanks. He was pretty mature for his age and would say the sweetest words… it was all talk, of course. I would’ve done a lot for that boy and he just kept hopping back and forth between me and the other girl. He knew I was so crazy about him that I was willing to overlook what he did. And I learned my lesson not to put a man before myself, my needs, my feelings, and my respect for myself.
I feel like you do love him so much, but we just worry that you’re clearly willing to accept behavior from him that will hurt you and that you’re clearly uncomfortable with for the sake of “love”. I don’t believe in all of the “we’re meant to be, I found my true soulmate” most of the time, honestly. Relationships are about love, respect, and standing by each other no matter what. Saying it’s “meant to be” takes out the fact that your love and your relationship is a choice- you choose to be with each other each and every day. Some days are harder than others, some seasons, some years.
I don’t think about my husband in terms of we are meant to be, instead I choose every day to love him, be there for him, and stay loyal to him. He does the same for me. We have gone through some tough times together already, but neither of us has ever been ready to say we’re done. It doesn’t make any sense to me to say “you are perfect for me and I am perfect for you but not right now, life got too shitty, it’s better for us to separate and figure this out alone because we can’t figure it out together… but obviously we are still perfect for one another and our relationship was written in the heavens”. I think you are confusing sacrificing in relationships with what is happening now. Basically saying “look how much I love and trust him, that I will allow him to sleep with other women so he can be happy” and him reassuring you that it means nothing and you will always be the only one for him and no one else could ever come close, etc etc. So it puts you in denial… you are broken up. He wants to sleep with other people. In words, you may have the most beautiful relationship. But as others have pointed out, there will be MANY trying times in life. If you were unable to navigate this together as a couple, it does not bode well for future problems- will you break up and see others every time your relationship gets rocky? This sets the precedent for that.
Also, I am really not trying to be cruel, but what will you do if one of his one night stands ends up pregnant? Though unlikely, it could happen. Will you move to Australia to help him raise his child with another woman? Will he move back to you and just ignore that child and maybe send money from time to time? Will he pressure her to take care of it?
Even if he doesn’t sleep with anyone and comes back to you, I still feel if it got bad enough for you to call it quits once, you just can’t say it will never happen again. I’m sure you guys never anticipated this, but you also can’t anticipate what the future will bring.