If they have a woman that is willing and wanting to marry them while they are locked up in prison, which would be dumb on the womans part, then yes they should be allowed to go ahead and get married if that is what they truly want to go through with.
Laws should be based on a reason that is goal-oriented. Preventing prisoners from getting married isn't goal-oriented, it's punitive. There's no benefit to society in forbidding convicted murderers from getting married. It's also unfair to the other party in the marriage. If someone is willing to marry a convicted murderer, they should be allowed to. There are also people convicted of murder who can be reformed.
I have no problem with convicted murderers such as Joran van der Sloot being allowed to marry while in prison. It's not like marriage is going to make their prison sentence shorter. And it's one less crazy chick out of the dating pool. If two sick people make each other happy, it spares the rest of us.
I do not believe convicted murderers such as Joran van der Sloot should be allowed to marry while in prison. The point of prison is to restrict rights and I think it is reasonable to take away the right to marry. I believe this could easily wait to their release or be forgone is they will be in prison for life.
They should not be allowed to marry in prison because they have committed a crime. They now should have less rights than a citizen of that country. They should sever their time and be punished. They should not get to have the privilege of marriage. No they should not get to do this.