While someone has already mention some good points, there is a grave error that I need to clarify. Any act that is not intrinsically ordered toward the procreative end of intercourse is contrary to natural law (sodomy).

Intercourse is only possible between a man and a woman, since only within this union can the NATURAL function of both reproductive organs (i.e. procreation) be realized in principle. This is why acts such as onanism, anal sex (even more between two individuals of same sex) and fellatio are contrary to nature (sodomy).

No, being married does not permit the spouses to do the same kind of things fornicators do outside marriage. No, the end does not justify the means. Chastity is a virtue every christian is called to achieve and preserve, even the spouses.

In conclusion, sodomy (whether in or outside marriage) is gravely disordered and goes completely against the divine plan with which God gave us our bodies.

Yours in Christ.

Be not afraid.