I've kept moderately quiet on this but thought I'd just toss out my opinion.
Yes - there are consequences to your not having a companion on the cruise. Is that a reason to take someone else's girlfriend? Not my call.
I know that here there have been discussions including myself on multi-dating and there are diverse opinions on that. My current opinion is no.
Also, Dawn and I are very much in the "old fashioned" camp where both she and I are uncomfortable with in my own situation of me dating someone who doesn't openly acknowledge me / isn't divorced. So I'm perhaps the Ashley Wilkes here (google may be your friend).
From where I sit - you are OM. If you and WG are happy with that, that's "fine". Is her boyfriend? Personally I can't see myself having intimacy with anyone who I'm not in a committed relationship with. And yes - I'm an old fart despite being a year younger than you.
But our paths have been very different. I got married as a young man - forever I presumed - and spent more than half my life in that paradigm. You got married later, for a shorter period of time and have been single for quite some time. I try not to apply my morals to others.
For me - I would not be dating someone else's girlfriend. Not that they are "property" but because there are norms in our society that I respect.
I'll set down my wiffle bat now.
On BD H52, W50 T27, M26 S21, D23 BD-9-Mar-16 D-15-Jan-18 Final-19-Apr-18 I am a storyteller. The story may do you no good. But a story is never for the listener. It is always for the one who tells