would honestly expect most other men to genuinely refuse this offer if actually presented with it.

That's more of an issue with making sure that the logistics work right - the prior for someone going around with a million dollars to spend to get straight men to sleep with them is so low that "this is obviously a scam or a ruse of some sort" eats up the remaining probability mass.

Imagine instead of some guy coming up to you with the offer, you get a phone call from your attorney. He says he is in the room with some %celebrity's attorney with a million dollar check on the table, and lays out the offer for you.