You will drive yourselves crazy with the equal thing - there is no such thing as equal. If everyone is keeping tabs on what is fair and balanced you will, similarly, drive yourselves crazy because everyone will be using a different accounting system. This is a no win game where everyone loses because everyone is too afraid to do anything for fear of making someone else mad.
I agree that "married wife" is different from "dating prospective wife", but the groundwork you lay now is the foundation upon which the future marriage will be built so it is important to get it right and have the right boundaries and expectations in place. You also need to figure out the "rules of combat" - how you discuss issues and iron them out as they arise.
I agree that there should be no sex before marriage. We all also know that it does happen. Truthfully it is not for you to police this one though. Either your husband is up to the task at hand or he is not. He is married to you. He is not a single man. He needs to be MORE responsible when other people are involved. If he is ready to have sex with her, but not ready to be married to her then he needs to address that internally and spiritually. There are some excellent men on here that will be more than willing to discuss it with him. If he is ready to be married to her then he should get on and get married to her.
As for you having 10 years of marriage and always being put first - she is right. She now needs to wrap herself around the idea that she is the shiny new thing on the block and she will always be put first too. If she tries to be you, she will fail miserably. Just like if you try to be her, you will fail miserably. If she tries to be the best that she can be, then she will always exceed you at the things she is good at. Just like you will exceed her in your areas of strength. Isn't that kind of the idea of polygamy? Everyone gets to play to their strengths? Everyone gets recognized for their contribution? So what if I never make the best housekeeper in the world - I can still kick your butt when it comes to math homework! This is not a competition. We are not trying to be better or more valuable than each other. We are trying to be better people today than we were yesterday. We are trying to compliment each other.
The very language you are using - fair, equal, "put first" - are setting you up for failure. How about - respected, valued, unique, individual. Focus on figuring out what peoples needs are and meet them. My SW (no longer in the family) kept insisting I go out on a date with my husband. It took me a while to figure out that she needed to go on a date. She felt if I went on one, then she would be entitled to one. I didn't need one - but she did. I was perfectly happy for her to go out on a date. On a regular basis. I did not feel the need to demand equal time/nights/money spent etc. This was something she needed and therefore it was important to me that it be met. What I did need was to be respected and heard for my unique needs as they arose - even if it was something that held little or no value to her.
Back to the "first wife" versus "second wife" and the "married wife" versus "dating wife" - the reality is that you and he have 10 years of common experiences and knowledge of each other. You have the upper hand in terms of knowing the ropes. She has the upper hand in bringing fresh ideas and perspective. You have the upper hand in being a known entity who will react in a known way in certain situations. She is more unpredictable but also bring a different set of life skills to the equation. Instead of fighting it and pretending that the dynamic is different from what it is maybe you should embrace it? first does not mean more important, but it does mean different. You all need to figure out what her role is going to be. That means you are going to have to give up some things (I volunteer right here right now to abdicate all future ironing responsibilities) and maybe some things you like to have done your way and that means she is going to have to realize that there are some things that are just done your (and his) way and she is going to have to yield - but it does not mean she lost, or that she is less important or that it is set in stone and cannot be re-visited if it isn't working - it just means that for right now we are going to do things the way they have always been done in this one little area of our life. Again - who is keeping tabs of how many things are done your way and how many are done her way and what accounting system are we using and on a scale of 1 to 10 what is the a more important thing to have done your way....arrggghhh - stop the madness - this will drive you crazy.
I like your idea of praying individually and together, I like the idea of reassuring her, it is VERY POSITIVE that she can express that she is feeling jealous, I would encourage everyone in the family to read this together and see what discussions arise. You guys seem like you are bringing up the issues and talking about them and communication really is the way through to the other side. Praying you all have the wisdom to figure out the "dating rules" you all can live with