If you'd said "Call me a traditionalist, but I'm not into casual sex and free love," that'd be one thing, traditionalism against the modern-day sexual revolution (in the 60s and 70s, but relatively modern-day). But to say "Call me a traditionalist, but I'm not into concubines," is ironic, because concubines are a tradition from thousands of years ago, not a modern invention. Rather, the word "concubine" sounds extraordinarily dated to modern ears, and brings to mind familial arrangements that would be repulsive to us today because of our more egalitarian views toward women. Men treating women as people rather than property (ala concubines) is more a modern concept than a traditional one, particularly in the grand scheme of history.
Further, most people today know the word "concubine" because it's in the Bible, and most of the time today when we hear someone talking about "tradition" in regards to sexuality, it's someone saying same-sex marriage should be illegal because Bible. Of course, the inherent contradiction between the people saying "The Bible defines marriage as one man and one woman" and the Bible itself defining marriage as "one man and multiple wives (just don't marry sisters like Jacob did, because that was awkward), and also multiple concubines and maidservants, and you can have a baby with the maidservant of one of your wives and it'll count as that wife's baby" exists anyway, but to hear someone saying "I believe in traditional marriage, not concubines!" kind of brings that contradiction to the forefront.
You, of course, were not saying anything about same-sex relationships, and this isn't meant to presume where your feelings lie on them. Words do have baggage, though, and those are the thoughts that went through my mind when I read your post. I get what you're saying, but using that word specifically kind of muddled your point in an amusing way. Monogamy is more traditional than having multiple partners in the context of the last 300 years or so of Western society ("these days"), but bringing the word (and hence the concept) "concubines" into the picture suddenly shifts the scope from a couple centuries of Western culture to several millennia of Western and Middle/Near Eastern culture, and from that vantage point, monogamy has not been the norm.