Handmaiden, it's obviously good to consider your partners feelings but i'm wondering what you hope to gain from this weekend party sex 'deal'?
Are you merely doing this to turn him on as suggested above?
Are you concerned that he might have a change of heart after the fact?
I say this as a mono male with partner seeking poly. I too get off on the 'fantasy' of my girlfriend having a rendevous with a random stranger but realised that in 'reality' I would be a little too concerned for her safety and the well being of our relationship to be comfortable. We agreed that it would make us both feel better if she got to know a potential partner rather than having a one-night stand. I guess it depends what you are looking for and as you say you are trying to figure out what you want.