Question: "What is a Christian couple allowed to do in sex?"
Answer: The Bible says that "Marriage should be honored by all, and the marriage bed kept pure, for God will judge the adulterer and all the sexually immoral" (Hebrews 13:4). Scripture never says what a husband and wife are or are not allowed to do sexually. Husbands and wives are instructed, "Do not deprive each other except by mutual consent and for a time (1 Corinthians 7:5a). This verse perhaps lays down the principle for sexual relations in marriage. Whatever is done, it should be mutually agreed upon. No one should be encouraged or coerced to do something he or she is uncomfortable with or thinks is wrong. If a husband and wife both agree that they want to try something (e.g., oral sex, different positions, sex toys, etc.), then the Bible does not give any reason why they cannot.
There are a few things, though, that are never allowable sexually for a married couple. The practice of "swapping," or "swinging/swingers," or "bringing in an extra" (threesomes, foursomes, etc.) is blatant adultery (Galatians 5:19; Ephesians 5:3; Colossians 3:5; 1 Thessalonians 4:3). Adultery is sin even if your spouse allows, approves, or even participates in it. Pornography appeals to the "lust of the flesh and the lust of the eyes" (1 John 2:16) and is therefore condemned by God as well. A husband and wife should never bring pornography into their sexual union. Other than these two items, there is nothing that Scripture explicitly forbids a husband and wife to do with each other as long as it is by mutual consent.