Queer is a reclaimed umbrella term for any non-cis or non-hetero relationship. If two men were in a relationship they might consider themselves queer. It’s not really a word you’d ascribe to other people, it’s self-descriptive.

A man and woman in a relationship is a hetero relationship, if one or both are trans or gender non-confirming they may consider themselves queer. It’s with noting that a hetero relationship does not necessarily mean they’re straight either, bi or pan people often date the opposite gender.

I wouldn’t normally consider a non-monogamous relationship queer by default, otherwise anyone who cheats would be queer.

Since it’s a reclaimed slur the best move is to not use it at all unless the person you’re talking about has made it clear they’re comfortable being described that way by you.