Is it really assur ? Please bring the source.
The Gemara in Pesachim 51a writes:
עִם הַכֹּל אָדָם רוֹחֵץ, חוּץ מֵאָבִיו וְחָמִיו וּבַעַל אִמּוֹ וּבַעַל אֲחוֹתוֹ
A person may bathe with anyone except for his father, and his father-in-law, and his mother’s husband, and his sister’s husband.
This is codified in Halacha in Shulchan Aruch, Even HaEzer 23:6:
עוד אסרו בגמ' לרחוץ עם אביו ואחיו ובעל אמו ובעל אחותו ונהגו עכשיו היתר בדבר הואיל ומכסין ערותן בבית המרחץ ליכא למיחש להרהורא (אגודה)
Furthermore, in the Talmud they forbade one to wash with his father or brother or mother's husband or sister's husband. But now they are accustomed to allow this since they cover their genitals in the bathhouse, we need not be concerned about sexual thoughts (Agudah).
Thus, when in a mikvah, which necessitates complete nudity to immerse, this would contravene this halacha.
Exploring this further, I came across the Din Online article which does note some leniencies. It writes there the following:
The Rema says that a father and son should not bath together, which would include going to the mikva together. The reason is because it will cause him improper thoughts. (This halacha also includes not bathing together with one’s father in law, step father, an brother in law). If they are wearing underwear it is permitted. There are poskim that look for reason to explain why many people are not careful about this, but they say that essentially one should be careful about this. There are however a few lenient factors brought in the poskim. A father may go to the mikva with a young child, (there are different opinions in the poskim as to the age, some say 9-10, others say until bar mitzvah) since he won’t have an issue of unwanted thoughts. Others say that if they are in different parts of the mikva and they might only meet for a very short time, then it is permitted.
Pesachim 51a, Rema Shulchan Aruch E:H 23-6, Aruch Hashulchan E:H 23-8, Likras Shabbos 1-chap. 7 ftnt 66, Lehoros Nossan 1-61, Minchas Yitzchok 4-62, Om Ani Choma (Chidosom Mishta’ashosh 187). Various Halachos that are applicable to Shabbos pg. 23, Chut Shani (Shabbos) 3 pg. 284, Derech Sicha 2 pg. 475, Ashrei Ish O:CH 2 1-22 ftnt 31.
Is there a reason without a Beis Hamikdash for a young child to go to the mikvah? If he's too young to have improper thoughts how can he be a baal keri?– HeshyJan 26 at 23:47