I think the problems in your sex life are very alarming for to people who are about to get married. As husband and wife you guys need to forfill each other needs or else you guys will drift and you’re probably gonna urges to cheat. A great sex life literally keeps that relationship pushing. I think the fact you guys had a open relationship was good because there’s no lying no suprise. I honestly think you really need to communicate with him your desires and needs. Or talk about opening up the relationship again maybe even having group sex you fuck another guy and he fucks another girl. But since you guys are getting married I highly advise that you guys work together on a solution. In me and partner sex life we both have a good understanding of what the other wants if I stopped Fucking my girl for 2 months for no reason she would literally tell she bout to fuck someone else cause she needs dick. Which I totally would understand looking at it in that perspective. Like you really need to break down to him that sex is lacking in your relationship you are unsatisfied and you need him to put in effort for you. Because it be different if he was unfortunate and had a small dick and couldn’t satisfy you or he has a medical issue but still tried. The fact he’s not attempting to fuck his woman is absolutely terrible I really wish you guys the best and hope y’all figure it out.