I think your very smart to consider there will more than likely be an end to any additional relationships to ur marriage.
I have known people who are poly their whole lives. And for the most part they manage a secondary relationship well and have made life long partners / friends.
however, I have been into polyamourous relationships and I've been married for 21 years as well. Both of those relationships ended and of course they still say they will always love me then will always be friends, but it still hurts. It also does take away from your primary relationship.its not supposed to take away from your primary relationship, however if there are problems with your secondary relationships who are you going to turn to? Probably your primary relationship which will put a strain on that.
currently, I am reconsidering if I should go back to a monogamous marriage with my husband. Being poly is very exciting and a lot of fun but you have to wonder if it's worth the pain. I'm sure many people on the site will disagree with me but it is a reality. If you're willing to be hurt and take a chance on losing faith in love then maybe it's worth it for you? You have to be the one to ultimately make that decision. Since you're already questioning that there may be an end to a secondary relationship maybe you have enough wisdom to not get too involved with people and risk getting hurt. If you can clearly separate you are feelings and set boundaries then maybe it is worth it for you.