Man I never knew that fundamentalist Christian married couples were allowed to have such kinky sex. It's interesting, Catholics have their "natural law" tradition that traces back to Aristotelian-Aquinas notions of "nature" and "reason" and sees "procreation" as the be all and end all of marital sex.
There are many (not all) fundamentalist Protestants who couldn't care one whit about such Aristotelian notions but rather care only about what the Bible says. Therefore, there is nothing wrong with contraception between married couples because the Bible doesn't forbid that (it does tell us to "multiply," but if a good Christian couple wants to use contraception to rationally plan how and when it will have children, so be it. Aquinas's notions of "naturalness" are not binding; Scripture is).
But anyway, it's surprising how many things apparently are allowed in "The Marital Bed" according to this site's literal interpretation of the Bible. For instance, sex toys:
If a sex toy is used, it should be as an enhancement, not the center of attention. If it brings the couple closer, and results in one or both of them enjoying sex more, that is good. If it results in either person feeling badly, that is not good.
The site also says that oral sex is okay and even hints that anal sex may be okay, but warns possible "health risks."
But this one really took me:
Tying a partner up can be very arousing for both of the one tied and the one who is not tied. It can show a sense of trust, and can allow the bound person to feel free to do nothing and just receive. If bondage is mutually enjoyed and not extreme, we see no reason not to make it a part of a couple's sex life.