I wonder if she wasn't feeling jealous that it's you he's bringing for valentine's, so she manipulated him to bringing her as well. Let him put his money where his mouth is. If the only reason he's buying her a cruise is out of pity, ask him to send her on a cruise by herself. You say she's expected to be on her own anyway, and if anything I'd say it's even lonelier to be by yourself when your partner is on the same boat than it is to be completely on your own...
I'm extremely bothered that he didn't tell you about his 5 (7?) other girlfrieands until he had let you fall in love with him. That's manipulative. I guess there's some leeway depending on how fast you fell in love and whether or not he led you to believe he was single. But I definitely think "I'm poly and I'm seeing other people" should definitely come up by the second date, preferably first. Monos don't need to waste time and energy on people they won't have any interest in being with.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."