The letter you are refering to is probably #43 written to Michael Tolkien 6-8 March 1941. (On the subject of marriage and relations between the sexes.)
At six pages of text, it is much too long for me to type out here, even if that was legal, which it isn't.
Therefore I suggest you check your library for a copy of The Letters of J.R.R. Tollkien edited by Humphrey Carpenter with the assistance of Christopher Tolkien. ISBN 0-618-05699-8
'Share and enjoy'