Waiting after using the bathroom before having relations

Question

Where is it brought down that one should wait after going to the bathroom and before having relations. I have heard this before and 2 reasons given. One is that because of the "ruach rah" in the bathroom that (I guess?) "attaches" itself to the private parts of ones body and therefore should avoid relations until a certain amount of time has past (next question.) And the other reason is because some sort of danger if the urine mixes together with semen. What are the sources for both reasons (if the 2nd one which I only heard once is valid.)

As well how much time is it that one should wait?

And is the only problem the "actual" relations or even the time the couple will spend together before hand.

Answer 1

The source for this is Gittin 70a

תנו רבנן הבא מבית הכםא אל ישמש מטתו עד שישהה שיעור חצי מיל מפני ששד בית הכםא מלוה עמו ואם שימש הוויין לו בנים נכפים

Soncino Translation:

The Rabbis taught: On coming from a privy a man should not have sexual intercourse till he has waited long enough to walk half a mil, because the demon of the privy is With him for that time; if he does, his children will be epileptic.

Regarding the amount of time mentioned - 'half a mil' -

S"A 459:2 writes that the mil is 18 minutes. Rama 261:1 agrees. Biur Halacha 459:2 s.v. Haviy quotes some who consider it 22.5 minutes and others who say that it is 24 minutes. see here

...so that makes half a mil at least 9 minutes.

See also Mishna Brurah Siman 240 S.K. 54 who brings the above Gemara and adds that the Gemara is only referring to a permanent bathroom [ie one with a seat] and not a temporary one.