Sex during periods is prohibited not just because of the blood, rather it is for many reasons as stated in the detailed answer below.

Sex during pregnancy is halal in Islam unless that will cause her harm, for it is haram for him to do anything that will harm her.

If a man has anal sex with his wife, she is not considered to be divorced as many people think, because there is no Shar`i evidence at all that indicates this. But the scholars said that if a man habitually does this, his wife has the right to ask for a divorce. The wife has to resist this evil action and remind her husband about Allah and about the punishment for transgressing the limits set by Allah.