My parents have over 300 rental properties. I have some experience in this area. With spot cleaning on older carpet you may lighten the carpet in the area you are cleaning enough that you may have to do an entire section but...... for just the spots.
Remove excess fecal matter by picking it up without rubbing it into the carpet. Attempt to remove as soon as possible. Use a spray commercial carpet cleaner made for pet stains. Test it in a closet first. Older carpets will sometimes bleach out and discolor badly. If it does not discolor, then spray the area well, feel free to spray in excess and place a hot wet cloth over the area for a while. The longer the better but keep the area very damp. If you do not have a carpet cleaning machine, I would suggest buying/renting one. Bissell and Hoover makes some really good ones for your type use. On your first clean, try to suck up as much of the area as possible. Spray with hot water several times and clean from out to center. Always clean toward the spot. If you have a handheld steamer, this may help if the spot is being stubborn. Steam the spot and use the carpet cleaner machine/shop vac often. Do not hold the steamer in one spot for a long time, it can do damage.
If the spots are being really stubborn, my parents also use a commercial cleaner, its about $90 a gallon, called Orange Clean (I think that is the name), It does a great job and is super concentrated. You should really do the entire carpet after removing the spot stains because generally, where there is feces, there is urine. The carpet pad has probably been soaked with urine and it will start to smell really bad as time passes. I would go over the carpet several times. Also, may sure to let the carpet dry out afterwards. You may want to open windows and use box fans or use a commercial blower. My parents use a Lasko floor blower, about $60 at Walmart. They move a lot of air. If a carpet has been soiled really bad, consider replacing it.
WHat Ive been trying to figure out is how to get it out of underwear. I mean your out on a date, everything is perfect, you have been extra charming the whole evening, and then you are getting a little comfortable back at the homestead, and then her eyes widen. Suffice to say its worse than a Hitchcock movie.
LOL Mr Jumps. If you're really serious, here's what you do. Soak the underwear in bleach water. Let it soak for awhile. Then put them with other whites in the washer and add bleach. Don't put anything that has color in it, because it might get bleach stains. Stick with whites only. I hope that helps.
Don't feel bad. It happens to a lot of people whether they want to admit it or not.
Here's a little-known secret I'm willing to share if you promise not to tell anyone else. Promise? Ok...
Saliva. Get the entire spot or streak wet with human saliva (dingo will also work but who raises dingos these days?) then flick slowly with a mildly-abrasive object such as the tip of your tongue. I guarantee you will NOT be bothered by that stain tomorrow.
Black underwear. Just buy only black undies and stains dont show. As for carpeted floors...Hot water and Oxyclean powder works like a charm. its my go to stain remover for every stain i encounter in my car, in laundry and for pet puke, pee, and poop on the carpet.