Bi M/F relationship and exploring sucking cock together

My partner and I are both bisexual. Lately we’ve been thinking of exploring a fantasy involving another man.

Has anyone here done something like this with a long-term partner? How did it go, and did reality match the fantasy?
Were there any boundaries or emotional considerations you wish you’d discussed beforehand?

We’re mainly interested in hearing honest experiences, lessons learned, and anything you wish you’d known before taking that step