I would consider it a dealbreaker for several reasons. One, you can never be open about the relationship, it by nature has to be in the closet. Bleh. Two, what if the betrayed partner finds out and violently freaks out and you are in the line of fire (hopefully not literally but sometimes people *really* take betrayal poorly... crimes of passion and all that). Three, if someone is willing to lie to their primary partner, the person who's supposed to be the most important person in their life, about something so big, how on earth could you be as naive to think they wouldn't lie to you about something important... like, say, STD status.
If I knew the person and knew there were in fact extenuating circumstances, I might consider a quick fuck. But dating? Hell no, for my own sake, not for any moral reasons... even though I don't think it's right, my concerns in this case are practical.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.