The only defense for this is working in retail makes you a captive audience when people create awkward situations. If someone decided to taunt them about being gay it could become very uncomfortable for them.

This doesn't sound like that but maybe they're projecting or reactive because of other negative interactions

That said, there are ways to shut down situations like this diplomatically. They are obviously uncomfortable discussing their relationship around you. Gray Rock them from now on, and don't gossip about them.

Leave it be.