[quote:2j8exukk]On the other hand, Catholics are not allowed to receive communion in other churches (except the Orthodox Churches under certain circumstances). There are several reasons for this, but I feel the most poignant reason I have heard is because we do not believe in fornication.[/quote:2j8exukk]
At first I was confused by this statement. I was like, “what Christian church does?” Then it dawned on me when you referred to marriage and the Eucharist so I just want to expand on that a little.

I believe what Benedict to be saying is this:
There quite a few similarities between communion and marriage. What we are really getting at is that when we partake of the Eucharist, we are joining ourselves to the body of Christ. We do that in the following ways:
[b:2j8exukk] ¬? [/b:2j8exukk]We are joined to Jesus Christ, himself, as we consume his flesh and drink his blood
[b:2j8exukk] ¬? [/b:2j8exukk]We are joined spiritually with all other believers who are also eating his flesh and drinking his blood
The parallels between the sacrament of the altar (communion or Eucharist) and Holy Matrimony are astounding. In marriage the renewal of those vows and the marriage covenant is done through sexual intercourse – the joining of the man and woman as one. There is an act of unification there.

However, if a man and a woman who are not married are having sexual intercourse then they are fornicating (a.k.a. pre-marital or extra-marital sex). They are doing something that is a sign of marriage without actually being married. Neither one of them professed their lifelong committment to each other before the community and before God in a marriage ritual, therefore they are not married and not permitted to engage in sexual intercourse.

When we eat Jesus’ body and drink his blood we are uniting ourselves to Jesus in the same way a married man and a married women unite themselves in sexual intercourse. While we are not having sex with Jesus the same type of bond is being formed between our God and his people. Partaking in the Eucharist is the act of renewing our covenant with God.

However, if you are not in good standing with God you may not receive the sacrament – you should reconcile with him first.

So, to make a long story short since non-Catholic Christians are not Catholic, they are separated brethren, they are not permitted to receive communion because they do not believe all the same things Catholics believe – especially about what is taking place in the Eucharist.

We believe that it is literally the Body and Blood, soul and divinity, of our Lord Jesus Christ whereas a non-Catholic does not. In that sense, for a non-Catholic to walk up and take communion would be to fornicate.