@John2563 I don't know what your minimum night stay is, but the only way you could really avoid requests like this is to have a 2 night minimum. Of course guests can always send an inquiry message asking for you to waive a 2 night minimum, just like they can ask for you to make an exception to anything else. Nothing to prevent that, just tell them you don't accept less than 2 night bookings, as stated in your listing.
As far as someone booking in order to have an affair, hosts have no assurance that even guests who book for 3 nights aren't cheating on their spouse. As far as I'm concerned, what consenting adults do is none of my business, but as a fellow home-share host, I fully understand not wanting a bedroom in your home to be booked solely for the purpose of sex. That's what hotel rooms are for.
I only host one guest at a time, so I don't have to deal with this sort of thing.