Negative Mitzvah 150
We are forbidden to eat Ma'aser Sheini while impure
Deuteronomy 26:14 "Neither have I consumed any part of it while impure"
This Negative Mitzvah applies to the manner of eating Ma'aser Sheini. (See Introduction to Negative Mitzvot 141-144.)
This is a portion of our harvest, set aside to be brought up to Jerusalem and eaten there. This Negative Mitzvah prohibits one who is impure from eating the Ma'aser Sheini.
Negative Mitzvah 151
An "Onan" is prohibited from eating Ma'aser Sheini
Deuteronomy 26:14 "I have not eaten of it in my mourning"
A person who has just suffered the loss of a close relative is prohibited from eating Ma'aser Sheini and must follow the Torah's rules for mourning.
From the day of the death until the burial, such a person is called an "Onan".
There are many detailed laws which apply to this first day of mourning. One of them concerns Ma'aser Sheini.
Ma'aser Sheini should be eaten in an atmosphere of joy and gratefulness, celebrating all that HaShem has given us.
Obviously, an "Onan" cannot ignore his feelings of sorrow. Therefore, he is forbidden to eat the Ma'aser Sheini.