OK, pretty much always a top man here, but now and then......:biggrin1:
Shower shot/shower enema attachment (with the flexible metal hose, smooth nozzle and adjustable pressure knob attached to the shower head - you know what I'm talking about?) is the most reliable cleansing that I've found - warm water flushing out the rectum/lower colon repeatedly until clear and clean. No problem in my shower having water just the right temperature - no scalding. I've not had a problem of retaining brown water at all, nor have most of my partners, if we've made sure to sit on the toilet and expel everything we could. Probably wise to start this a little earlier than right before you're pulling off each other's underwear , though! You could always put a bath towel down under you, just in case....saves the sheets or bedcovers!
Other than buying the shower enema attachment, purchase a drug store/chemist's rubber douche bottle with a long nozzle (or even the Fleet brand or store brand enemas which are handy and do well) -- just keep flushing out, as others have said here.
A light meal before the date might help not feeling "bloated" for later sexual pressures <wink> -- but has nothing to do with a clean fuck, really, IMO (It usually takes a lot longer for ingested food to work its way through the digestive tract than that.)
Metamucil fiber powder drunk nightly is great advice -- a good buddy of mine uses it like clockwork to keep the bowels firm and easy to eject. Good fiber-filled diet itself is one of the main aids to helping keep things cleaned out.
Go forth, squeaky clean folks, and have fun!