Q - Can a person receive communion at mass if they have been married outside of the church but are planning on getting married in the church in the future?
A - Thanks for your question! First off, the answer depends on the circumstances surrounding the issue that aren't provided.
I am assuming (forgive me if I am wrong) that you are speaking of a situation where someone has been married in a civil ceremony and then plans on having the marriage "blessed" in the Catholic Church. This process of having a marriage "blessed" is called convalidation. This is a statement by the Church that the marriage is in fact valid. If this is the case, then the Catholics involved in such a situation are not allowed to receive Communion until the marriage is convalidated. Now, this does not mean the Church wants to give them the boot or leave them out. She still invites them to participate as fully as possible. They are invited to come to Mass, but they are not able to receive Communion until the marriage is recognized by the Church.
The reason for these rules has to do with what the Church sees in the marriage bond. The Church has the greatest respect for marriage (as it does for all Sacraments). In maintaining this respect, she wants to make sure that we are respecting all marriage unions, which means that she must examine the situation of a marriage outside of the Church with great detail to see if it is valid.
If there are irregular circumstances which means the Church cannot recognize the marriage presently, she asks that the couple would refrain from partaking in Communion, because the bond of unity between members of the Body of Christ are in question. This is due to the fact that Catholics are bound to follow what is called "canonical form", which means the law of the Church regarding the circumstances of a marriage. Since form is under question, the Church asks that the couple refrain from receiving the Eucharist until it can be confirmed that their marriage is valid. This is because we want to make sure we are receiving the Eucharist with the proper reverance to Christ and in the state of grace.
I think the focus needs to be on the end, not the process. The end is getting everything straightened up with the Church. The process may not necessarily be fun, but it can also be a chance to grow closer to Christ.
To have a marriage convalidated it is a relatively simple process. First, the couple should approach a deacon or priest to work with them. He will help guide the couple through the process. Second, you will have to provide baptismal records and fill out forms providing the Church with information about the spouses. Third, if there was a previous marriage it is necessary to seek a declaration of nullity (commonly called an anullment). Fourth, the spouses will celebrate the Sacrament of Reconciliation. Fifth, there is a simple ceremony of recognition.
If this is not the situation that you are speaking of, please let me know.