Well... has it been billed as a romantic getaway- or just a trip that a lot of rich people are taking together who all happen to be married and are all mutually friends? Personally I wouldn't call being on holiday with a partner's friends especially romantic, going away as a couple alone certainly would be.

I think maybe you need to shift your perspective on this. If they're all well off, what looks like luxury (and ergo romantic) for you may be standard for them! Also, looking at your partner's age it might just be the fact that he (and his friends) are older and settled into committed relationships make the standard "mates holiday" look a lot different to the ones you'd go on in your early 20s.

Ignore the standard comments about cheating- I don't think any man is dumb enough to take the other woman on a group holiday.