The gemora says that if the chatan is an avel then he has relations and only later does he sit shiva. They call this biah d'mitzvah- but if he is an onen- then he isn't metzuveh so what is going on?
daniel, Hollywood, FL USA
Strictly speaking, the Chasan ought to bury his dead and wait seven days, until the period of mourning terminates, before performing Chupah and Bi'ah. Only, because of the tremendous loss involved, Chazal gave him a special dispensation to postpone the burial (in which case he does not become an Onen yet) until after the Chupah and the Be'ilas Mitzvah. As a result, the seven days of festivity, having begun before the period of mourning, will precede the seven days of mourning.
Bear in mind also that Aninus, in areas not related to Kodshim, is only mid'Rabanan.
Be'Virchas Kol Tuv.
i believe in parenthese you mean he does not become an "ovel" yet
The questioner only asked about Be'ilas Mitzvah, so I confined my answer to same.
The Chachamim removed from them the obligation to bury the deceased until after the Be'ilas Mitzvah, effectively postponing the Aninus. Aveilus would not apply anyway as long as the deceased has not been buried.
After the Be'ilas Mitzvah, the Aninus takes effect, obligating them to bury the deceased. Following that, the Aveilus ought to come into effect, only, since the seven days of Sheva Berachos have already begun, the Aveilus too, is postponed until after the conclusion of the Sheva Berachos.
Be'Virchas Kol Tuv