I am a social progressive and maintain that taboo is societal construction that shouldn't govern your love life... that aside I personally think that I would be unable to be with a person not dedicated to me, and I wouldn't be able to dedicate myself to more than one person
My aunt is in a relationship like that. She has a husband and a boyfriend. We talked about it a bit when she stayed the night after a cross-country trek to take her daughter to pick up the daughter's kids and bring them back here before driving back to Los Vegas. For some reason plenty of our family will have nothing to do with her now despite the fact it doesn't affect them in the slightest. My opinion on it though? I'd never be comfortable in a relationship like that. That doesn't mean it's wrong for someone else to be in one so long as everyone involved is fine with it. Is it weird? Sure, if you consider anything too out of the ordinary weird. Is it wrong though? Not at all so long as no one ends up getting hurt because of it.