QUESTION: What does it mean to defile the marriage bed? Is there anything sexually that should or should not be done?
ANSWER: The scripture you are referring to in regard to what it means to defile the bed of a married couple is found in Hebrews 13. The chapter contains a variety of admonitions by the apostle Paul. In verse 4 he admonishes that marriage be considered honorable by all people and that "the marital union be undefiled" (the KJV has "and the bed undefiled"). The Greek word translated "undefiled" is amiantos, Strong's Concordance #G283, which literally means unsoiled or pure. The last part of verse 4 is a warning that those who do "defile their bed," like adulterers, will be judged by God.
To understand what it means to have an undefiled martial relationship, we need to know what can defile it. The Old Testament has plenty to say regarding how a person can pollute themselves through sex. God tells us NOT to commit adultery with another person's mate (Exodus 20:14, Leviticus 18:20). We are also commanded not to have intercourse with animals (Leviticus 18:23). Sex between those of the same gender are also expressly forbidden (verse 22).
Avoiding these practices would not only save a person from experiencing the negative consequences they bring, they would keep (for those who are married) the "marriage bed" or sexual relationship with their partner pure.
The New Testament book of 1Corinthians, the seventh chapter, gives us a principle or two regarding the proper sexual relationship within marriage. Both the husband and the wife need to seriously consider their mate's sexual needs and their capabilities. Each person should regard their bodies as belonging to their mate and not under their own total control to do with it whatever they please. Sex should be refrained from only by mutual agreement for the purpose of fasting and prayer, after which time the couple should come together sexually so that they are not tempted to fulfill their needs elsewhere (1Corinthians 7: 1 - 5).
Aside from the above on how not to defile the marriage bed, the Bible does not put any restrictions on how a couple perform sexually.