Mildly Infuriating

In case you are serious or someone else is serious about this:
Masks, just like underwear, are permeable. If they wouldn't be, you would suffocate as no air would be able to reach your lungs. (At your rear, that's just a consequence of low requirements towards the material, which mainly serves as a covering and being able to dry.) So gas molecules are usually not stopped to pass through your underwear or masks.

The main difference between, e.g. FFP2/3, masks and your underwear is the size of particles that are able to pass through. Your underwear will be able to catch some moisture of a juicy fart and probably some shit particles. However, most of the gases and smaller particles can pass through. Filter masks are able to stop most particles down to a certain size. For FFP 2 masks at least 94% of particles down to a size of 0,6 micrometers can be stopped. Particles smaller than that are likely to pass through. FFP 3 masks block at least 99% of particles down to the same size.

The thing with viruses like Covid-19 is, that they are usually expelled from an infected person via the respiratory system, that means, your mouth and nose. There, the virus cells are mostly soluted within the moist air you breathe out. Moist air means: there are very tiny (mainly) water particles which are spread over a certain volume of gas. And they are floating with the currents of the gas. That's what basically constitutes an aerosol. Particles, tiny and lightweight enough to float with a gas (for a while).
You can see that clearly with spray cans. With breathing air it's not so good to see with the naked eye.
Those moist particles, loaded with virus cells, are usually large enough to be filtered out by FFP masks. It's not perfect of course, but it can help a great deal and contribute to prevention measures.

The stinky stuff in farts is afaik a pure gas and not an aerosol. (Did you know that in terms of gas volume, just about 2 % of a single fart contains the smelly chemicals?) Which is why neither masks, nor underwear help so much against stinky farts.

FFP masks are mandatory in jobs where dangerous smaller particles can enter your body. They help against dust, smoke and a lot of aerosols. But they don't filter out pure gases. That's what gas masks are for.

In terms of infection spreading, a neat side-effect of masks is that they can slow down the velocity of exhaled air, which helps to reduce how far the aersol spreads around a person. Trying to blow out a candle while wearing a mask isn't as easy as without a mask, which demonstrates this effect.

Futhermore, there has been a study suggesting that wearing a mask for a while or wearing it during rainy weather can - up to a certain point - further improve the filter capabilities of such face masks, since the moisture accumulates on the surface of the mask (in- and/or outside) and by this forms another protective barrier.

Thanks for coming to my TED talk.