Monogamy isn't dead. It is still the leading form of relationship in D/s and SM relationships, no matter what someone else might tell you. Human nature dictates that we aren't wired for long term fidelity, but so many of us are perfectly fine in one partner for all time relationship. You can still look for monogamy if that is what you want.
On a few sites, I frequent I have been getting an impression recently that poly relationships and playing with others outside your primary relationship are not only accepted but expected. I'm uncomfortable with this way of thinking. As children and teens many of us dream of finding that one for us to love and cherish and be with forever. Rarely do we imagine that our one's come with others. It's not wrong to want a poly relationship, but it's not always welcomed.
This is the group of people I want to talk to. You do not have to accept a poly relationship if you do not want one. Just because you may select to be submissive does not automatically determine that you share your Dominant with however many others s/he wishes to have. When you put together you wants and needs of a relationship make sure you know what kind of relationship you want.
Do you want monogamy? Then say so. If you are comfortable with your partner engaging in SM play with others make sure you voice that as well. How about sex? Will there be multiple sex partners? Know this stuff before you commit yourself to anyone.
And know it about yourself too. If you can't see yourself in a monogamous relationship, make sure your partners know this up front.
A common question I get from readers of SubmissiveGuide is how to handle and accept that their Dominant has asked to play with others besides themselves. It's not an easy question to answer because I too have had to ask myself this very question. What I decided may not work for those that ask me, but I do my best.
I don't see SM play the same way I see sex. Sexual intimacy is important to me as a solid expression of our love and commitment. SM doesn't work that way for me. While SM has intimacy to it, it is an exploration of fantasy and dark desires. Many fantasies we have can not be played out with the person we love, and so we need someone else to explore those paths. I trust that whatever limits I place on my Dominant in regard to play with others that he will respect them. With this trust, I am okay with my Dominant having play partners other than myself. I am secure in my position as his love and submissive and don't feel threatened by his other play partners.
This may not be the same for you, but it is something you need to know before you make a decision to tell your Dominant what you want and what you don't. If you can't handle your Dominant playing with others, don't just accept it because you think that's the best thing to do. You'll be miserable and a relationship isn't mean to make you feel miserable. Do the best thing for yourself and seek happiness, in whatever form that may be.
Monogamy is something to embrace if it feels right, no matter which way the current trend seems to be leading.