Something I was wondering last night, is when is the right moment to do it? For example, I know that one safe time is like 3-4 hours after going to the bathroom, because the rectum naturally pushes all the fecal mater upwards and leaves itself relatively clean. But another thing to consider is the time when the stool starts forming after one eats. How much time will you recommend?
PS: I know all this can be dealt with with a douche, but I just wanted to know.
I think almost none of us even think about that because sex happens in the moment.
I don’t even ask if she’s empty, full or whatever. I tell her I want to fuck her and until now she never said no.
Our experience is similar to @Backdoorlover where she doesn’t really worry about the timing of eating and anal sex. Common sense applies, and it probably wouldn’t be wise for her to eat spicy or greasy foods the night before, but those aren’t foods my wife likes to eat anyway.
Some women follow a strict diet prior to anal,
and people who engage in deep anal play may want to watch their diet but for most of us this isn’t necessary.
For a drama free anal experience, a water or saline enema and cleansing is all that’s really needed. The rectum is separated from the lower colon by the sigmoid bend. There’s absolutely no need to flush the lower colon unless you want to stick a 12” or longer toy up your partner’s ass.
My wife constantly worries that she’s cleaned out enough for anal sex, although it’s actually gotten better recently. Now, when we discover dirty sheets during anal, we continue to fuck instead of her getting up and trying to clean out. A little “dirt” doesn’t bother me at all and can be a bit of a turn on and reminder that I’m in her “naughty hole”.
It took her a few years to perfect her clean out routine much of that was due to her fear of not being completely clean.
In summary, because I’m long-winded, no need to fast before anal, no need to flush the lower colon, and absolutely no need to freak out about encountering a little poo during the most erotic form of sex I’ve experienced. Hope this helps answer your question.
Or I could use a condom, an thus I also reduce the risk of getting an STD.
They are cheap, easy to get and come in many different presentations.
To get an STD you must have sex with someone who already has an STD. Monogamous people are highly unlikely to have an STD.
I wonder why you’re always worrying about all kinds of things that aren’t even on your path yet.
If you plan on having any kind of penetrative sex with a new person I recommend you use a condom. Her cleaning out won’t prevent the possibility of contracting a STD.
If you’re in a long term relationship, you can enjoy anal with no condom. The sensation of anal is just too good to spoil by wearing a condom in a steady relationship. Encountering feces during anal doesn’t bother us (took her a while) so I’ll never wear a condom. It’s a remarkably pleasurable feeling. Fortunately for me, it is for her too.
So it’s best to use a condom early in a newer sexual relationship. It wasn’t as risky when I was younger as most STD’s could be easily treated with antibiotics.
So I guess it would be something like: if you are always thinking about all the ways a trip may go wrong, then you won’t enjoy it. And if something goes wrong, don’t overreact. I have to stop being an overthinker, but it’s hard.
Still, if the moment ever gets, I’ll make sure to use condom just in case (unless she begs me not, in that case I wouldn’t say no).
Keep in mind anal sex is different in many ways. At least it causes a deeper connection because trust is key.
So overthinking is the enemy of fun anal sex. Relaxation and enjoyment is where it’s at. I can imagine it’s not as simple as turn off a switch, to stop your overthinking. It’s a slow process ,but you need to start it sooner better then later. 
If it would be a person you just met and don’t know enough, always use condom. And if such a person begs you to not use it, that is a big red flag. Run away or use 2 condoms over itself 
(just joking, you should not use 2 condoms over itself).
Ah yes, because she might be one of those persons that like to transmit STDs on purpose.
Also, two condoms may be really uncomfortable and may tear appart.
Yeah, I’ll have that in mind for when the time comes.