Nope you are not wrong. We are programed in our society to feel that marriage lends us the time and exclusive body rights of our spouse. Poly isn't wrong either some people can love more than one person and it not take away from any of the relationships.
Monogamy supposes that one person can and should meet all the needs of another.
Polyamory supposes that one person can't possibly meet all the needs of another person and that love is a limitless resource that only grows as time goes on.
My hus and I have been together 16 years married 13 of that and we have had lovers over 9 of those years and we love each other more now than ever. It can work but it takes communication, honesty with kindness, and self-responsibility. Think carefully and give yourself time. Jealousy does happen and can be overcome. There may be issues of all sorts but love means you work it out. It's up to you and he to decide. Share your fears honestly I think you will be ok.
As for your second question it is entirely possible that he loves you and wants you to be his primary but you have to decide together what ground rules if any there should be before you get married. Hope this helps.