Yeah, my thing is I essentially agree with what everyone says here as being a common trend or rule - once a cheater always - but I don't think that is always the case - my instincts would tell me not to give them the benefit of the doubt, but I think there would be certain people in certain situations that could be made exceptions for. Certain things and motivations need to be there, although I'm not entirely clear on what that is.
Anyhow, yes, monogamy is absolutely possible in our quadra. I think betas want committed relationships deep down, but we just get bored quickly, or dissatisfied, or maybe even lose belief that monogamy is worth it. Some come around, others don't.