I am assuming here that the hypothetical situation involves a partner in a monogamous relationship who would be upset if their partner was intimate with another person. Assuming that situation, my take on it is slightly different than the others.
Whilst I agree that embarking on a relationship with a person who has an unconsenting partner is likely to have an unpleasant impact, I feel that the person who is cheating in that relationship is the cheater and nobody else. That person is an adult, and is fully and solely responsible for their choice to cheat and their choice to act on that decision to cheat on their partner. If one person wouldn't sleep with them, then they're just as likely to find another. Personally, I cannot and will not blame them all for the transgressions of one.
I think that blaming the person they're using to cheat on their partner with is counter productive and besides the point. If we're going to blame some people for the transgressions of others, we might as well blame the partner cheated on as well. I think that blaming anybody else is passing the buck away from the cheater, and I'm sure the cheater is only too happy to do that by blaming others for their own choices.
And furthermore, if the person who is intimate with the person with an unconsenting partner is single, then they are cheating on nobody. They have made no promises to the other person's partner, so I consider this a matter for their personal ethics. I refuse to condemn a person for cheating when they're actually single and it's their partner who is doing the cheating.
Having said that, I personally think that in general embarking on relationships with people with unconsenting partners is a bad idea and can frequently lead to more trouble than it's worth. I can't tell you how tired I am of hearing "My wife doesn't understand me" or some such nonsense, which I find most usually translates into "No matter how much I try to convince her to celebrate it, my wife insists on deploring and disliking my inconsiderate self--absorbed behaviour". I personally feel that a person who cheats is not a person I can relate to well, but that's my personal choice and I would never blame anybody for their partner's misdeeds.
I strongly feel that as individuals, we each need to be fully and solely accountable and responsible for our own behaviour and nobody else's. In short, that means that ethically I cannot blame one person for another's behaviour, and that means I cannot blame one person for their partner's transgressions.
I would recommend extreme caution to anybody considering embarking on a relationship like this because in my experience they usually lead only to unpleasantness.